diff --git a/doc/python_api/examples/bpy.msgbus.1.py b/doc/python_api/examples/bpy.msgbus.1.py
index 8164272d521..c33d5788ddf 100644
--- a/doc/python_api/examples/bpy.msgbus.1.py
+++ b/doc/python_api/examples/bpy.msgbus.1.py
@@ -17,6 +17,13 @@ The following updates do **not** trigger message bus notifications:
 - Moving objects in the 3D Viewport.
 - Changes performed by the animation system.
 
+Changes done from ``msgbus`` callbacks are not included in related undo steps,
+so users can easily skip their effects by using Undo followed by Redo.
+
+Unlike properties ``update`` callbacks, message bus update callbacks are postponed
+until all operators have finished executing.
+Additionally, for each property the callback is only triggered once per update cycle,
+even if the property was changed multiple times during that period.
 
 Example Use
 -----------
diff --git a/source/blender/animrig/intern/bone_collections.cc b/source/blender/animrig/intern/bone_collections.cc
index e965f0e224a..9f49dfca029 100644
--- a/source/blender/animrig/intern/bone_collections.cc
+++ b/source/blender/animrig/intern/bone_collections.cc
@@ -63,8 +63,6 @@ BoneCollection *ANIM_bonecoll_new(const char *name)
   STRNCPY_UTF8(bcoll->name, name);
   bcoll->flags = default_flags;
 
-  bcoll->prop = nullptr;
-
   return bcoll;
 }
 
@@ -76,6 +74,9 @@ void ANIM_bonecoll_free(BoneCollection *bcoll, const bool do_id_user_count)
   if (bcoll->prop) {
     IDP_FreeProperty_ex(bcoll->prop, do_id_user_count);
   }
+  if (bcoll->system_properties) {
+    IDP_FreeProperty_ex(bcoll->system_properties, do_id_user_count);
+  }
   MEM_delete(bcoll);
 }
 
@@ -258,6 +259,10 @@ static BoneCollection *copy_and_update_ownership(const bArmature *armature_dst,
     bcoll->prop = IDP_CopyProperty_ex(bcoll_to_copy->prop,
                                       0 /*do_id_user ? 0 : LIB_ID_CREATE_NO_USER_REFCOUNT*/);
   }
+  if (bcoll->system_properties) {
+    bcoll->system_properties = IDP_CopyProperty_ex(
+        bcoll_to_copy->system_properties, 0 /*do_id_user ? 0 : LIB_ID_CREATE_NO_USER_REFCOUNT*/);
+  }
 
   /* Remap the bone pointers to the given armature, as `bcoll_to_copy` is
    * assumed to be owned by another armature. */
@@ -1406,6 +1411,10 @@ blender::Map<BoneCollection *, BoneCollection *> ANIM_bonecoll_array_copy_no_mem
       bcoll_dst->prop = IDP_CopyProperty_ex(bcoll_src->prop,
                                             do_id_user ? 0 : LIB_ID_CREATE_NO_USER_REFCOUNT);
     }
+    if (bcoll_src->system_properties) {
+      bcoll_dst->system_properties = IDP_CopyProperty_ex(
+          bcoll_src->system_properties, do_id_user ? 0 : LIB_ID_CREATE_NO_USER_REFCOUNT);
+    }
 
     (*bcoll_array_dst)[i] = bcoll_dst;
 
@@ -1425,6 +1434,9 @@ void ANIM_bonecoll_array_free(BoneCollection ***bcoll_array,
     if (bcoll->prop) {
       IDP_FreeProperty_ex(bcoll->prop, do_id_user);
     }
+    if (bcoll->system_properties) {
+      IDP_FreeProperty_ex(bcoll->system_properties, do_id_user);
+    }
 
     /* This will usually already be empty, because the passed BoneCollection
      * list is usually from ANIM_bonecoll_listbase_copy_no_membership().
diff --git a/source/blender/blenkernel/BKE_armature.hh b/source/blender/blenkernel/BKE_armature.hh
index ad4fde98ed9..3da78d9a3d2 100644
--- a/source/blender/blenkernel/BKE_armature.hh
+++ b/source/blender/blenkernel/BKE_armature.hh
@@ -38,6 +38,13 @@ struct EditBone {
   EditBone *next, *prev;
   /** User-Defined Properties on this Bone */
   IDProperty *prop;
+  /**
+   * System-Defined Properties storage.
+   *
+   * In Blender 4.5, only used to ensure forward compatibility with 5.x blendfiles, and data
+   * management consistency.
+   */
+  IDProperty *system_properties;
   /**
    * Edit-bones have a one-way link  (i.e. children refer
    * to parents.  This is converted to a two-way link for
diff --git a/source/blender/blenkernel/intern/action.cc b/source/blender/blenkernel/intern/action.cc
index a693e9197e3..8e362743d31 100644
--- a/source/blender/blenkernel/intern/action.cc
+++ b/source/blender/blenkernel/intern/action.cc
@@ -1340,6 +1340,9 @@ void BKE_pose_copy_data_ex(bPose **dst,
     if (pchan->prop) {
       pchan->prop = IDP_CopyProperty_ex(pchan->prop, flag);
     }
+    if (pchan->system_properties) {
+      pchan->system_properties = IDP_CopyProperty_ex(pchan->system_properties, flag);
+    }
 
     pchan->draw_data = nullptr; /* Drawing cache, no need to copy. */
 
@@ -1537,6 +1540,10 @@ void BKE_pose_channel_free_ex(bPoseChannel *pchan, bool do_id_user)
     IDP_FreeProperty_ex(pchan->prop, do_id_user);
     pchan->prop = nullptr;
   }
+  if (pchan->system_properties) {
+    IDP_FreeProperty_ex(pchan->system_properties, do_id_user);
+    pchan->system_properties = nullptr;
+  }
 
   /* Cached data, for new draw manager rendering code. */
   MEM_SAFE_FREE(pchan->draw_data);
@@ -1664,13 +1671,21 @@ void BKE_pose_channel_copy_data(bPoseChannel *pchan, const bPoseChannel *pchan_f
 
   /* id-properties */
   if (pchan->prop) {
-    /* unlikely but possible it exists */
+    /* Unlikely, but possible that it exists. */
     IDP_FreeProperty(pchan->prop);
     pchan->prop = nullptr;
   }
   if (pchan_from->prop) {
     pchan->prop = IDP_CopyProperty(pchan_from->prop);
   }
+  if (pchan->system_properties) {
+    /* Unlikely, but possible that it exists. */
+    IDP_FreeProperty(pchan->system_properties);
+    pchan->system_properties = nullptr;
+  }
+  if (pchan_from->system_properties) {
+    pchan->system_properties = IDP_CopyProperty(pchan_from->system_properties);
+  }
 
   /* custom shape */
   pchan->custom = pchan_from->custom;
@@ -2074,6 +2089,8 @@ void BKE_pose_blend_write(BlendWriter *writer, bPose *pose, bArmature *arm)
     if (chan->prop) {
       IDP_BlendWrite(writer, chan->prop);
     }
+    /* Never write system_properties in Blender 4.5, will be reset to `nullptr` by reading code (by
+     * the matching call to #BLO_read_struct). */
 
     BKE_constraint_blend_write(writer, &chan->constraints);
 
@@ -2138,6 +2155,8 @@ void BKE_pose_blend_read_data(BlendDataReader *reader, ID *id_owner, bPose *pose
 
     BLO_read_struct(reader, IDProperty, &pchan->prop);
     IDP_BlendDataRead(reader, &pchan->prop);
+    BLO_read_struct(reader, IDProperty, &pchan->system_properties);
+    IDP_BlendDataRead(reader, &pchan->system_properties);
 
     BLO_read_struct(reader, bMotionPath, &pchan->mpath);
     if (pchan->mpath) {
diff --git a/source/blender/blenkernel/intern/armature.cc b/source/blender/blenkernel/intern/armature.cc
index 5479e287ae7..41a9dbd0ba8 100644
--- a/source/blender/blenkernel/intern/armature.cc
+++ b/source/blender/blenkernel/intern/armature.cc
@@ -109,6 +109,9 @@ static void copy_bone_collection(bArmature *armature_dst,
   if (bcoll_dst->prop) {
     bcoll_dst->prop = IDP_CopyProperty_ex(bcoll_dst->prop, lib_id_flag);
   }
+  if (bcoll_dst->system_properties) {
+    bcoll_dst->system_properties = IDP_CopyProperty_ex(bcoll_dst->system_properties, lib_id_flag);
+  }
 
   /* Bone references. */
   BLI_duplicatelist(&bcoll_dst->bones, &bcoll_dst->bones);
@@ -226,6 +229,11 @@ static void armature_foreach_id_bone(Bone *bone, LibraryForeachIDData *data)
       data, IDP_foreach_property(bone->prop, IDP_TYPE_FILTER_ID, [&](IDProperty *prop) {
         BKE_lib_query_idpropertiesForeachIDLink_callback(prop, data);
       }));
+  BKE_LIB_FOREACHID_PROCESS_FUNCTION_CALL(
+      data,
+      IDP_foreach_property(bone->system_properties, IDP_TYPE_FILTER_ID, [&](IDProperty *prop) {
+        BKE_lib_query_idpropertiesForeachIDLink_callback(prop, data);
+      }));
 
   LISTBASE_FOREACH (Bone *, curbone, &bone->childbase) {
     BKE_LIB_FOREACHID_PROCESS_FUNCTION_CALL(data, armature_foreach_id_bone(curbone, data));
@@ -238,6 +246,12 @@ static void armature_foreach_id_editbone(EditBone *edit_bone, LibraryForeachIDDa
       data, IDP_foreach_property(edit_bone->prop, IDP_TYPE_FILTER_ID, [&](IDProperty *prop) {
         BKE_lib_query_idpropertiesForeachIDLink_callback(prop, data);
       }));
+  BKE_LIB_FOREACHID_PROCESS_FUNCTION_CALL(
+      data,
+      IDP_foreach_property(
+          edit_bone->system_properties, IDP_TYPE_FILTER_ID, [&](IDProperty *prop) {
+            BKE_lib_query_idpropertiesForeachIDLink_callback(prop, data);
+          }));
 }
 
 static void armature_foreach_id_bone_collection(BoneCollection *bcoll, LibraryForeachIDData *data)
@@ -246,6 +260,11 @@ static void armature_foreach_id_bone_collection(BoneCollection *bcoll, LibraryFo
       data, IDP_foreach_property(bcoll->prop, IDP_TYPE_FILTER_ID, [&](IDProperty *prop) {
         BKE_lib_query_idpropertiesForeachIDLink_callback(prop, data);
       }));
+  BKE_LIB_FOREACHID_PROCESS_FUNCTION_CALL(
+      data,
+      IDP_foreach_property(bcoll->system_properties, IDP_TYPE_FILTER_ID, [&](IDProperty *prop) {
+        BKE_lib_query_idpropertiesForeachIDLink_callback(prop, data);
+      }));
 }
 
 static void armature_foreach_id(ID *id, LibraryForeachIDData *data)
@@ -283,6 +302,8 @@ static void write_bone(BlendWriter *writer, Bone *bone)
   if (bone->prop) {
     IDP_BlendWrite(writer, bone->prop);
   }
+  /* Never write system_properties in Blender 4.5, will be reset to `nullptr` by reading code (by
+   * the matching call to #BLO_read_struct). */
 
   /* Write Children */
   LISTBASE_FOREACH (Bone *, cbone, &bone->childbase) {
@@ -300,6 +321,8 @@ static void write_bone_collection(BlendWriter *writer, BoneCollection *bcoll)
   if (bcoll->prop) {
     IDP_BlendWrite(writer, bcoll->prop);
   }
+  /* Never write system_properties in Blender 4.5, will be reset to `nullptr` by reading code (by
+   * the matching call to #BLO_read_struct). */
 
   BLO_write_struct_list(writer, BoneCollectionMember, &bcoll->bones);
 }
@@ -356,8 +379,11 @@ static void armature_blend_write(BlendWriter *writer, ID *id, const void *id_add
 static void direct_link_bones(BlendDataReader *reader, Bone *bone)
 {
   BLO_read_struct(reader, Bone, &bone->parent);
+
   BLO_read_struct(reader, IDProperty, &bone->prop);
   IDP_BlendDataRead(reader, &bone->prop);
+  BLO_read_struct(reader, IDProperty, &bone->system_properties);
+  IDP_BlendDataRead(reader, &bone->system_properties);
 
   BLO_read_struct(reader, Bone, &bone->bbone_next);
   BLO_read_struct(reader, Bone, &bone->bbone_prev);
@@ -377,6 +403,8 @@ static void direct_link_bone_collection(BlendDataReader *reader, BoneCollection
 {
   BLO_read_struct(reader, IDProperty, &bcoll->prop);
   IDP_BlendDataRead(reader, &bcoll->prop);
+  BLO_read_struct(reader, IDProperty, &bcoll->system_properties);
+  IDP_BlendDataRead(reader, &bcoll->system_properties);
 
   BLO_read_struct_list(reader, BoneCollectionMember, &bcoll->bones);
   LISTBASE_FOREACH (BoneCollectionMember *, member, &bcoll->bones) {
@@ -538,6 +566,9 @@ void BKE_armature_bonelist_free(ListBase *lb, const bool do_id_user)
     if (bone->prop) {
       IDP_FreeProperty_ex(bone->prop, do_id_user);
     }
+    if (bone->system_properties) {
+      IDP_FreeProperty_ex(bone->system_properties, do_id_user);
+    }
     BLI_freelistN(&bone->runtime.collections);
     BKE_armature_bonelist_free(&bone->childbase, do_id_user);
   }
@@ -551,6 +582,9 @@ void BKE_armature_editbonelist_free(ListBase *lb, const bool do_id_user)
     if (edit_bone->prop) {
       IDP_FreeProperty_ex(edit_bone->prop, do_id_user);
     }
+    if (edit_bone->system_properties) {
+      IDP_FreeProperty_ex(edit_bone->system_properties, do_id_user);
+    }
     BLI_remlink_safe(lb, edit_bone);
     MEM_freeN(edit_bone);
   }
@@ -571,6 +605,9 @@ static void copy_bonechildren(Bone *bone_dst,
   if (bone_src->prop) {
     bone_dst->prop = IDP_CopyProperty_ex(bone_src->prop, flag);
   }
+  if (bone_src->system_properties) {
+    bone_dst->system_properties = IDP_CopyProperty_ex(bone_src->system_properties, flag);
+  }
 
   /* Clear the runtime cache of the collection relations, these will be
    * reconstructed after the entire armature duplication is done. Don't free,
diff --git a/source/blender/blenkernel/intern/grease_pencil_convert_legacy.cc b/source/blender/blenkernel/intern/grease_pencil_convert_legacy.cc
index 9945c712ea8..919c7173fab 100644
--- a/source/blender/blenkernel/intern/grease_pencil_convert_legacy.cc
+++ b/source/blender/blenkernel/intern/grease_pencil_convert_legacy.cc
@@ -1036,6 +1036,7 @@ static void legacy_gpencil_to_grease_pencil(ConversionData &conversion_data,
   BLI_assert(!grease_pencil.id.properties);
   if (gpd.id.properties) {
     grease_pencil.id.properties = IDP_CopyProperty(gpd.id.properties);
+    grease_pencil.id.system_properties = IDP_CopyProperty(gpd.id.properties);
   }
 
   /** Convert Grease Pencil data flag. */
diff --git a/source/blender/blenkernel/intern/layer.cc b/source/blender/blenkernel/intern/layer.cc
index f8895dca065..9514f1362d7 100644
--- a/source/blender/blenkernel/intern/layer.cc
+++ b/source/blender/blenkernel/intern/layer.cc
@@ -270,6 +270,9 @@ void BKE_view_layer_free_ex(ViewLayer *view_layer, const bool do_id_user)
   if (view_layer->id_properties) {
     IDP_FreeProperty_ex(view_layer->id_properties, do_id_user);
   }
+  if (view_layer->system_properties) {
+    IDP_FreeProperty_ex(view_layer->system_properties, do_id_user);
+  }
 
   MEM_SAFE_FREE(view_layer->object_bases_array);
 
@@ -503,6 +506,10 @@ void BKE_view_layer_copy_data(Scene *scene_dst,
   if (view_layer_dst->id_properties != nullptr) {
     view_layer_dst->id_properties = IDP_CopyProperty_ex(view_layer_dst->id_properties, flag);
   }
+  if (view_layer_dst->system_properties != nullptr) {
+    view_layer_dst->system_properties = IDP_CopyProperty_ex(view_layer_dst->system_properties,
+                                                            flag);
+  }
   BKE_freestyle_config_copy(
       &view_layer_dst->freestyle_config, &view_layer_src->freestyle_config, flag);
 
@@ -2395,6 +2402,8 @@ void BKE_view_layer_blend_write(BlendWriter *writer, const Scene *scene, ViewLay
   if (view_layer->id_properties) {
     IDP_BlendWrite(writer, view_layer->id_properties);
   }
+  /* Never write system_properties in Blender 4.5, will be reset to `nullptr` by reading code (by
+   * the matching call to #BLO_read_struct). */
 
   LISTBASE_FOREACH (FreestyleModuleConfig *, fmc, &view_layer->freestyle_config.modules) {
     BLO_write_struct(writer, FreestyleModuleConfig, fmc);
@@ -2454,6 +2463,8 @@ void BKE_view_layer_blend_read_data(BlendDataReader *reader, ViewLayer *view_lay
 
   BLO_read_struct(reader, IDProperty, &view_layer->id_properties);
   IDP_BlendDataRead(reader, &view_layer->id_properties);
+  BLO_read_struct(reader, IDProperty, &view_layer->system_properties);
+  IDP_BlendDataRead(reader, &view_layer->system_properties);
 
   BLO_read_struct_list(reader, FreestyleModuleConfig, &(view_layer->freestyle_config.modules));
   BLO_read_struct_list(reader, FreestyleLineSet, &(view_layer->freestyle_config.linesets));
diff --git a/source/blender/blenkernel/intern/lib_id.cc b/source/blender/blenkernel/intern/lib_id.cc
index 3e9af31dc14..772d3379cbc 100644
--- a/source/blender/blenkernel/intern/lib_id.cc
+++ b/source/blender/blenkernel/intern/lib_id.cc
@@ -943,6 +943,8 @@ static void id_swap(Main *bmain,
     /* Exception: IDProperties. */
     id_a->properties = id_b_back.properties;
     id_b->properties = id_a_back.properties;
+    id_a->system_properties = id_b_back.system_properties;
+    id_b->system_properties = id_a_back.system_properties;
     /* Exception: recalc flags. */
     id_a->recalc = id_b_back.recalc;
     id_b->recalc = id_a_back.recalc;
@@ -1609,6 +1611,9 @@ void BKE_libblock_copy_in_lib(Main *bmain,
   if (id->properties) {
     new_id->properties = IDP_CopyProperty_ex(id->properties, copy_data_flag);
   }
+  if (id->system_properties) {
+    new_id->system_properties = IDP_CopyProperty_ex(id->system_properties, copy_data_flag);
+  }
 
   /* This is never duplicated, only one existing ID should have a given weak ref to library/ID. */
   new_id->library_weak_reference = nullptr;
@@ -2617,6 +2622,8 @@ void BKE_id_blend_write(BlendWriter *writer, ID *id)
   if (id->properties && !ELEM(GS(id->name), ID_WM)) {
     IDP_BlendWrite(writer, id->properties);
   }
+  /* Never write system_properties in Blender 4.5, will be reset to `nullptr` by reading code (by
+   * the matching call to #BLO_read_struct). */
 
   BKE_animdata_blend_write(writer, id);
 
diff --git a/source/blender/blenkernel/intern/lib_id_delete.cc b/source/blender/blenkernel/intern/lib_id_delete.cc
index 6e33aeb08d6..1f465e2d56f 100644
--- a/source/blender/blenkernel/intern/lib_id_delete.cc
+++ b/source/blender/blenkernel/intern/lib_id_delete.cc
@@ -52,6 +52,11 @@ void BKE_libblock_free_data(ID *id, const bool do_id_user)
     MEM_freeN(id->properties);
     id->properties = nullptr;
   }
+  if (id->system_properties) {
+    IDP_FreePropertyContent_ex(id->system_properties, do_id_user);
+    MEM_freeN(id->system_properties);
+    id->system_properties = nullptr;
+  }
 
   if (id->override_library) {
     BKE_lib_override_library_free(&id->override_library, do_id_user);
diff --git a/source/blender/blenkernel/intern/lib_query.cc b/source/blender/blenkernel/intern/lib_query.cc
index aa09f502d02..4dba33d9681 100644
--- a/source/blender/blenkernel/intern/lib_query.cc
+++ b/source/blender/blenkernel/intern/lib_query.cc
@@ -393,6 +393,14 @@ static bool library_foreach_ID_link(Main *bmain,
       return false;
     }
 
+    IDP_foreach_property(id->system_properties, IDP_TYPE_FILTER_ID, [&](IDProperty *prop) {
+      BKE_lib_query_idpropertiesForeachIDLink_callback(prop, &data);
+    });
+    if (BKE_lib_query_foreachid_iter_stop(&data)) {
+      library_foreach_ID_data_cleanup(&data);
+      return false;
+    }
+
     AnimData *adt = BKE_animdata_from_id(id);
     if (adt) {
       BKE_animdata_foreach_id(adt, &data);
diff --git a/source/blender/blenkernel/intern/object.cc b/source/blender/blenkernel/intern/object.cc
index 939ebc55a04..90b44f7855b 100644
--- a/source/blender/blenkernel/intern/object.cc
+++ b/source/blender/blenkernel/intern/object.cc
@@ -428,6 +428,12 @@ static void object_foreach_id(ID *id, LibraryForeachIDData *data)
           data, IDP_foreach_property(pchan->prop, IDP_TYPE_FILTER_ID, [&](IDProperty *prop) {
             BKE_lib_query_idpropertiesForeachIDLink_callback(prop, data);
           }));
+      BKE_LIB_FOREACHID_PROCESS_FUNCTION_CALL(
+          data,
+          IDP_foreach_property(
+              pchan->system_properties, IDP_TYPE_FILTER_ID, [&](IDProperty *prop) {
+                BKE_lib_query_idpropertiesForeachIDLink_callback(prop, data);
+              }));
 
       BKE_LIB_FOREACHID_PROCESS_IDSUPER(data, pchan->custom, IDWALK_CB_USER);
       BKE_LIB_FOREACHID_PROCESS_FUNCTION_CALL(
diff --git a/source/blender/blenkernel/intern/paint.cc b/source/blender/blenkernel/intern/paint.cc
index ccd69f2639f..5444a2eb058 100644
--- a/source/blender/blenkernel/intern/paint.cc
+++ b/source/blender/blenkernel/intern/paint.cc
@@ -135,6 +135,7 @@ static void palette_undo_preserve(BlendLibReader * /*reader*/, ID *id_new, ID *i
    *       fairly delicate. */
   BKE_lib_id_swap(nullptr, id_new, id_old, false, 0);
   std::swap(id_new->properties, id_old->properties);
+  std::swap(id_new->system_properties, id_old->system_properties);
 }
 
 IDTypeInfo IDType_ID_PAL = {
diff --git a/source/blender/blenkernel/intern/pose_backup.cc b/source/blender/blenkernel/intern/pose_backup.cc
index 53fa1575113..cd51935adcd 100644
--- a/source/blender/blenkernel/intern/pose_backup.cc
+++ b/source/blender/blenkernel/intern/pose_backup.cc
@@ -36,7 +36,9 @@ struct PoseChannelBackup {
 
   bPoseChannel olddata; /* Backup of pose channel. */
   IDProperty *oldprops; /* Backup copy (needs freeing) of pose channel's ID properties. */
-  const Object *owner;  /* The object to which this pose channel belongs. */
+  /* Backup copy (needs freeing) of pose channel's system IDProperties. */
+  IDProperty *old_system_properties;
+  const Object *owner; /* The object to which this pose channel belongs. */
 };
 
 struct PoseBackup {
@@ -84,6 +86,9 @@ static void pose_backup_create(const Object *ob,
     if (pchan->prop) {
       chan_bak->oldprops = IDP_CopyProperty(pchan->prop);
     }
+    if (pchan->system_properties) {
+      chan_bak->old_system_properties = IDP_CopyProperty(pchan->system_properties);
+    }
 
     BLI_addtail(&pose_backup.backups, chan_bak);
     backed_up_bone_names.add_new(bone_name);
@@ -163,6 +168,9 @@ void BKE_pose_backup_restore(const PoseBackup *pbd)
     if (chan_bak->oldprops) {
       IDP_SyncGroupValues(chan_bak->pchan->prop, chan_bak->oldprops);
     }
+    if (chan_bak->old_system_properties) {
+      IDP_SyncGroupValues(chan_bak->pchan->system_properties, chan_bak->old_system_properties);
+    }
 
     /* TODO: constraints settings aren't restored yet,
      * even though these could change (though not that likely) */
@@ -179,6 +187,9 @@ void BKE_pose_backup_free(PoseBackup *pbd)
     if (chan_bak->oldprops) {
       IDP_FreeProperty(chan_bak->oldprops);
     }
+    if (chan_bak->old_system_properties) {
+      IDP_FreeProperty(chan_bak->old_system_properties);
+    }
     BLI_freelinkN(&pbd->backups, chan_bak);
   }
   MEM_freeN(pbd);
diff --git a/source/blender/blenkernel/intern/scene.cc b/source/blender/blenkernel/intern/scene.cc
index 240d7bb86af..d7a56c078fd 100644
--- a/source/blender/blenkernel/intern/scene.cc
+++ b/source/blender/blenkernel/intern/scene.cc
@@ -815,6 +815,9 @@ static bool strip_foreach_member_id_cb(Strip *strip, void *user_data)
   IDP_foreach_property(strip->prop, IDP_TYPE_FILTER_ID, [&](IDProperty *prop) {
     BKE_lib_query_idpropertiesForeachIDLink_callback(prop, data);
   });
+  IDP_foreach_property(strip->system_properties, IDP_TYPE_FILTER_ID, [&](IDProperty *prop) {
+    BKE_lib_query_idpropertiesForeachIDLink_callback(prop, data);
+  });
   LISTBASE_FOREACH (StripModifierData *, smd, &strip->modifiers) {
     FOREACHID_PROCESS_IDSUPER(data, smd->mask_id, IDWALK_CB_USER);
   }
@@ -873,6 +876,12 @@ static void scene_foreach_id(ID *id, LibraryForeachIDData *data)
         IDP_foreach_property(view_layer->id_properties, IDP_TYPE_FILTER_ID, [&](IDProperty *prop) {
           BKE_lib_query_idpropertiesForeachIDLink_callback(prop, data);
         }));
+    BKE_LIB_FOREACHID_PROCESS_FUNCTION_CALL(
+        data,
+        IDP_foreach_property(
+            view_layer->system_properties, IDP_TYPE_FILTER_ID, [&](IDProperty *prop) {
+              BKE_lib_query_idpropertiesForeachIDLink_callback(prop, data);
+            }));
 
     BKE_view_layer_synced_ensure(scene, view_layer);
     LISTBASE_FOREACH (Base *, base, BKE_view_layer_object_bases_get(view_layer)) {
@@ -1840,6 +1849,9 @@ Scene *BKE_scene_duplicate(Main *bmain, Scene *sce, eSceneCopyMethod type)
     if (sce->id.properties) {
       sce_copy->id.properties = IDP_CopyProperty(sce->id.properties);
     }
+    if (sce->id.system_properties) {
+      sce_copy->id.system_properties = IDP_CopyProperty(sce->id.system_properties);
+    }
 
     BKE_sound_destroy_scene(sce_copy);
 
diff --git a/source/blender/blenloader/intern/readfile.cc b/source/blender/blenloader/intern/readfile.cc
index 47a55163c03..64d5c89fb60 100644
--- a/source/blender/blenloader/intern/readfile.cc
+++ b/source/blender/blenloader/intern/readfile.cc
@@ -2118,6 +2118,11 @@ static void direct_link_id_common(BlendDataReader *reader,
     IDP_BlendDataRead(reader, &id->properties);
   }
 
+  if (id->system_properties) {
+    BLO_read_struct(reader, IDProperty, &id->system_properties);
+    IDP_BlendDataRead(reader, &id->system_properties);
+  }
+
   id->flag &= ~ID_FLAG_INDIRECT_WEAK_LINK;
 
   /* NOTE: It is important to not clear the recalc flags for undo/redo.
@@ -3089,6 +3094,12 @@ static void do_versions(FileData *fd, Library *lib, Main *main)
   /* Don't allow versioning to create new data-blocks. */
   main->is_locked_for_linking = true;
 
+  /* Code ensuring conversion from new 'system IDProperties' in 5.0. This needs to run before any
+   * other data versioning. Otherwise, things like Cycles versioning code cannot work as expected.
+   *
+   * Merge (with overwrite) future system properties storage into current IDProperties. */
+  version_forward_compat_system_idprops(main);
+
   if (G.debug & G_DEBUG) {
     char build_commit_datetime[32];
     time_t temp_time = main->build_commit_timestamp;
diff --git a/source/blender/blenloader/intern/versioning_450.cc b/source/blender/blenloader/intern/versioning_450.cc
index bf3a32fa94b..be3f61b7c89 100644
--- a/source/blender/blenloader/intern/versioning_450.cc
+++ b/source/blender/blenloader/intern/versioning_450.cc
@@ -33,6 +33,9 @@
 #include "BKE_customdata.hh"
 #include "BKE_fcurve.hh"
 #include "BKE_grease_pencil.hh"
+#include "BKE_idprop.hh"
+#include "BKE_lib_id.hh"
+#include "BKE_lib_query.hh"
 #include "BKE_main.hh"
 #include "BKE_mesh_legacy_convert.hh"
 #include "BKE_node.hh"
@@ -55,6 +58,60 @@
 
 // static CLG_LogRef LOG = {"blo.readfile.doversion"};
 
+void version_forward_compat_system_idprops(Main *bmain)
+{
+  auto idprops_process = [](IDProperty **idprops, IDProperty *system_idprops) -> void {
+    if (system_idprops) {
+      /* Other ID pointers have not yet been relinked, do not try to access them for refcounting.
+       */
+      if (*idprops) {
+        IDP_MergeGroup_ex(*idprops, system_idprops, true, LIB_ID_CREATE_NO_USER_REFCOUNT);
+      }
+      else {
+        *idprops = IDP_CopyProperty_ex(system_idprops, LIB_ID_CREATE_NO_USER_REFCOUNT);
+      }
+    }
+  };
+
+  ID *id_iter;
+  FOREACH_MAIN_ID_BEGIN (bmain, id_iter) {
+    idprops_process(&id_iter->properties, id_iter->system_properties);
+  }
+  FOREACH_MAIN_ID_END;
+
+  LISTBASE_FOREACH (Scene *, scene, &bmain->scenes) {
+    LISTBASE_FOREACH (ViewLayer *, view_layer, &scene->view_layers) {
+      idprops_process(&view_layer->id_properties, view_layer->system_properties);
+    }
+
+    if (scene->ed != nullptr) {
+      blender::seq::for_each_callback(&scene->ed->seqbase,
+                                      [&idprops_process](Strip *strip) -> bool {
+                                        idprops_process(&strip->prop, strip->system_properties);
+                                        return true;
+                                      });
+    }
+  }
+
+  LISTBASE_FOREACH (Object *, object, &bmain->objects) {
+    if (!object->pose) {
+      continue;
+    }
+    LISTBASE_FOREACH (bPoseChannel *, pchan, &object->pose->chanbase) {
+      idprops_process(&pchan->prop, pchan->system_properties);
+    }
+  }
+
+  LISTBASE_FOREACH (bArmature *, armature, &bmain->armatures) {
+    for (BoneCollection *bcoll : armature->collections_span()) {
+      idprops_process(&bcoll->prop, bcoll->system_properties);
+    }
+    LISTBASE_FOREACH (Bone *, bone, &armature->bonebase) {
+      idprops_process(&bone->prop, bone->system_properties);
+    }
+  }
+}
+
 static void version_fix_fcurve_noise_offset(FCurve &fcurve)
 {
   LISTBASE_FOREACH (FModifier *, fcurve_modifier, &fcurve.modifiers) {
diff --git a/source/blender/blenloader/intern/versioning_common.hh b/source/blender/blenloader/intern/versioning_common.hh
index e14dbff0565..2657880abb8 100644
--- a/source/blender/blenloader/intern/versioning_common.hh
+++ b/source/blender/blenloader/intern/versioning_common.hh
@@ -194,6 +194,10 @@ void version_update_node_input(
     FunctionRef<void(bNode *, bNodeSocket *, bNode *, bNodeSocket *)> update_input_link);
 
 bNode *version_eevee_output_node_get(bNodeTree *ntree, int16_t node_type);
+
+/* Allow 4.5 to open 5.0+ files and recover their system-defined ID properties. */
+void version_forward_compat_system_idprops(Main *bmain);
+
 bool all_scenes_use(Main *bmain, const blender::Span<const char *> engines);
 
 /* Adjust the values of the given FCurve key frames by applying the given function. The function is
diff --git a/source/blender/editors/armature/armature_add.cc b/source/blender/editors/armature/armature_add.cc
index 02f9ef9202b..442b8e169a6 100644
--- a/source/blender/editors/armature/armature_add.cc
+++ b/source/blender/editors/armature/armature_add.cc
@@ -1048,6 +1048,9 @@ static void copy_pchan(EditBone *src_bone, EditBone *dst_bone, Object *src_ob, O
   if (src_bone->prop) {
     dst_bone->prop = IDP_CopyProperty(src_bone->prop);
   }
+  if (src_bone->system_properties) {
+    dst_bone->system_properties = IDP_CopyProperty(src_bone->system_properties);
+  }
 
   /* Lets duplicate the list of constraints that the
    * current bone has.
@@ -1909,6 +1912,7 @@ static wmOperatorStatus armature_subdivide_exec(bContext *C, wmOperator *op)
 
       newbone->flag |= BONE_CONNECTED;
       newbone->prop = nullptr;
+      newbone->system_properties = nullptr;
 
       /* correct parent bones */
       LISTBASE_FOREACH (EditBone *, tbone, arm->edbo) {
diff --git a/source/blender/editors/armature/armature_intern.hh b/source/blender/editors/armature/armature_intern.hh
index e052f917367..47f1a83cc37 100644
--- a/source/blender/editors/armature/armature_intern.hh
+++ b/source/blender/editors/armature/armature_intern.hh
@@ -164,6 +164,7 @@ struct tPChanFCurveLink {
 
   /** copy of custom properties at start of operator (to be restored before each modal step) */
   IDProperty *oldprops;
+  IDProperty *old_system_properties;
 };
 
 /* ----------- */
diff --git a/source/blender/editors/armature/armature_relations.cc b/source/blender/editors/armature/armature_relations.cc
index 3c59ffa5654..69a27cb3937 100644
--- a/source/blender/editors/armature/armature_relations.cc
+++ b/source/blender/editors/armature/armature_relations.cc
@@ -291,6 +291,9 @@ static BoneCollection *join_armature_remap_collection(
   if (bcoll->prop) {
     new_bcoll->prop = IDP_CopyProperty_ex(bcoll->prop, 0);
   }
+  if (bcoll->system_properties) {
+    new_bcoll->system_properties = IDP_CopyProperty_ex(bcoll->system_properties, 0);
+  }
 
   bone_collection_by_name.add(bcoll->name, new_bcoll);
   return new_bcoll;
diff --git a/source/blender/editors/armature/armature_utils.cc b/source/blender/editors/armature/armature_utils.cc
index e6b3873312b..7d15a4e9efb 100644
--- a/source/blender/editors/armature/armature_utils.cc
+++ b/source/blender/editors/armature/armature_utils.cc
@@ -109,6 +109,9 @@ void bone_free(bArmature *arm, EditBone *bone)
   if (bone->prop) {
     IDP_FreeProperty(bone->prop);
   }
+  if (bone->system_properties) {
+    IDP_FreeProperty(bone->system_properties);
+  }
 
   /* Clear references from other edit bones. */
   LISTBASE_FOREACH (EditBone *, ebone, arm->edbo) {
@@ -523,6 +526,9 @@ static EditBone *make_boneList_recursive(ListBase *edbo,
     if (curBone->prop) {
       eBone->prop = IDP_CopyProperty(curBone->prop);
     }
+    if (curBone->system_properties) {
+      eBone->system_properties = IDP_CopyProperty(curBone->system_properties);
+    }
 
     BLI_addtail(edbo, eBone);
 
@@ -744,6 +750,9 @@ void ED_armature_from_edit(Main *bmain, bArmature *arm)
     if (eBone->prop) {
       newBone->prop = IDP_CopyProperty(eBone->prop);
     }
+    if (eBone->system_properties) {
+      newBone->system_properties = IDP_CopyProperty(eBone->system_properties);
+    }
   }
 
   /* Fix parenting in a separate pass to ensure ebone->bone connections are valid at this point.
@@ -797,6 +806,9 @@ void ED_armature_edit_free(bArmature *arm)
         if (eBone->prop) {
           IDP_FreeProperty(eBone->prop);
         }
+        if (eBone->system_properties) {
+          IDP_FreeProperty(eBone->system_properties);
+        }
         BLI_freelistN(&eBone->bone_collections);
       }
 
@@ -831,6 +843,9 @@ void ED_armature_ebone_listbase_free(ListBase *lb, const bool do_id_user)
     if (ebone->prop) {
       IDP_FreeProperty_ex(ebone->prop, do_id_user);
     }
+    if (ebone->system_properties) {
+      IDP_FreeProperty_ex(ebone->system_properties, do_id_user);
+    }
 
     BLI_freelistN(&ebone->bone_collections);
 
@@ -850,6 +865,10 @@ void ED_armature_ebone_listbase_copy(ListBase *lb_dst, ListBase *lb_src, const b
       ebone_dst->prop = IDP_CopyProperty_ex(ebone_dst->prop,
                                             do_id_user ? 0 : LIB_ID_CREATE_NO_USER_REFCOUNT);
     }
+    if (ebone_dst->system_properties) {
+      ebone_dst->system_properties = IDP_CopyProperty_ex(
+          ebone_dst->system_properties, do_id_user ? 0 : LIB_ID_CREATE_NO_USER_REFCOUNT);
+    }
     ebone_src->temp.ebone = ebone_dst;
     BLI_addtail(lb_dst, ebone_dst);
   }
diff --git a/source/blender/editors/armature/pose_transform.cc b/source/blender/editors/armature/pose_transform.cc
index a5338bc8f0f..4c011c1279e 100644
--- a/source/blender/editors/armature/pose_transform.cc
+++ b/source/blender/editors/armature/pose_transform.cc
@@ -767,6 +767,15 @@ static bPoseChannel *pose_bone_do_paste(Object *ob,
       pchan->prop = IDP_CopyProperty(chan->prop);
     }
   }
+  if (chan->system_properties) {
+    /* Same logic as above for system IDProperties, for now. */
+    if (pchan->system_properties) {
+      IDP_SyncGroupValues(pchan->system_properties, chan->system_properties);
+    }
+    else {
+      pchan->system_properties = IDP_CopyProperty(chan->system_properties);
+    }
+  }
 
   return pchan;
 }
@@ -1397,6 +1406,9 @@ static wmOperatorStatus pose_clear_user_transforms_exec(bContext *C, wmOperator
         if (pchan->prop) {
           IDP_FreeProperty(pchan->prop);
         }
+        if (pchan->system_properties) {
+          IDP_FreeProperty(pchan->system_properties);
+        }
       }
 
       /* was copied without constraints */
diff --git a/source/blender/editors/armature/pose_utils.cc b/source/blender/editors/armature/pose_utils.cc
index 1bfcfa8a67d..f95e0bac54b 100644
--- a/source/blender/editors/armature/pose_utils.cc
+++ b/source/blender/editors/armature/pose_utils.cc
@@ -226,6 +226,7 @@ static void fcurves_to_pchan_links_get(ListBase &pfLinks, Object &ob, bPoseChann
   /* Make copy of custom properties. */
   if (pchan.prop && (transFlags & ACT_TRANS_PROP)) {
     pfl->oldprops = IDP_CopyProperty(pchan.prop);
+    pfl->old_system_properties = IDP_CopyProperty(pchan.system_properties);
   }
 }
 
@@ -362,6 +363,9 @@ void poseAnim_mapping_reset(ListBase *pfLinks)
     if (pfl->oldprops) {
       IDP_SyncGroupValues(pfl->pchan->prop, pfl->oldprops);
     }
+    if (pfl->old_system_properties) {
+      IDP_SyncGroupValues(pfl->pchan->system_properties, pfl->old_system_properties);
+    }
   }
 }
 
diff --git a/source/blender/freestyle/intern/blender_interface/BlenderStrokeRenderer.cpp b/source/blender/freestyle/intern/blender_interface/BlenderStrokeRenderer.cpp
index c180d62de8b..485dd77f039 100644
--- a/source/blender/freestyle/intern/blender_interface/BlenderStrokeRenderer.cpp
+++ b/source/blender/freestyle/intern/blender_interface/BlenderStrokeRenderer.cpp
@@ -118,6 +118,10 @@ BlenderStrokeRenderer::BlenderStrokeRenderer(Render *re, int render_count)
   if (old_scene->id.properties) {
     freestyle_scene->id.properties = IDP_CopyProperty_ex(old_scene->id.properties, 0);
   }
+  if (old_scene->id.system_properties) {
+    freestyle_scene->id.system_properties = IDP_CopyProperty_ex(old_scene->id.system_properties,
+                                                                0);
+  }
   // Copy eevee render settings.
   BKE_scene_copy_data_eevee(freestyle_scene, old_scene);
 
diff --git a/source/blender/makesdna/DNA_ID.h b/source/blender/makesdna/DNA_ID.h
index afba5498f27..8fd37c284bf 100644
--- a/source/blender/makesdna/DNA_ID.h
+++ b/source/blender/makesdna/DNA_ID.h
@@ -442,6 +442,16 @@ typedef struct ID {
 
   IDProperty *properties;
 
+  /**
+   * System-defined custom properties storage.
+   *
+   * In Blender 4.5, only used to ensure forward compatibility with 5.x blendfiles, and data
+   * management consistency.
+   */
+  IDProperty *system_properties;
+
+  void *_pad1;
+
   /** Reference linked ID which this one overrides. */
   IDOverrideLibrary *override_library;
 
diff --git a/source/blender/makesdna/DNA_action_types.h b/source/blender/makesdna/DNA_action_types.h
index dd8ecb76551..e665f760a5b 100644
--- a/source/blender/makesdna/DNA_action_types.h
+++ b/source/blender/makesdna/DNA_action_types.h
@@ -287,6 +287,14 @@ typedef struct bPoseChannel {
   /** User-Defined Properties on this PoseChannel. */
   IDProperty *prop;
 
+  /**
+   * System-defined custom properties storage.
+   *
+   * In Blender 4.5, only used to ensure forward compatibility with 5.x blendfiles, and data
+   * management consistency.
+   */
+  IDProperty *system_properties;
+
   /** Constraints that act on this PoseChannel. */
   ListBase constraints;
   char name[/*MAXBONENAME*/ 64];
@@ -421,6 +429,8 @@ typedef struct bPoseChannel {
 
   BoneColor color; /* MUST be named the same as in Bone and EditBone structs. */
 
+  void *_pad2;
+
   /** Runtime data (keep last). */
   struct bPoseChannel_Runtime runtime;
 } bPoseChannel;
diff --git a/source/blender/makesdna/DNA_armature_types.h b/source/blender/makesdna/DNA_armature_types.h
index b4e060e82f7..c00ab2d097b 100644
--- a/source/blender/makesdna/DNA_armature_types.h
+++ b/source/blender/makesdna/DNA_armature_types.h
@@ -61,6 +61,14 @@ typedef struct Bone {
   struct Bone *next, *prev;
   /** User-Defined Properties on this Bone. */
   IDProperty *prop;
+  /**
+   * System-Defined Properties storage.
+   *
+   * In Blender 4.5, only used to ensure forward compatibility with 5.x blendfiles, and data
+   * management consistency.
+   */
+  IDProperty *system_properties;
+  void *_pad0;
   /** Parent (IK parent if appropriate flag is set). */
   struct Bone *parent;
   /** Children. */
@@ -276,6 +284,13 @@ typedef struct BoneCollection {
 
   /** Custom properties. */
   struct IDProperty *prop;
+  /**
+   * Custom system IDProperties.
+   *
+   * In Blender 4.5, only used to ensure forward compatibility with 5.x blendfiles, and data
+   * management consistency.
+   */
+  struct IDProperty *system_properties;
 
 #ifdef __cplusplus
   /**
diff --git a/source/blender/makesdna/DNA_layer_types.h b/source/blender/makesdna/DNA_layer_types.h
index 474547fc7b2..ef2c3d87d84 100644
--- a/source/blender/makesdna/DNA_layer_types.h
+++ b/source/blender/makesdna/DNA_layer_types.h
@@ -178,6 +178,13 @@ typedef struct ViewLayer {
   struct World *world_override;
   /** Equivalent to datablocks ID properties. */
   struct IDProperty *id_properties;
+  /**
+   * Equivalent to datablocks system-defined ID properties.
+   *
+   * In Blender 4.5, only used to ensure forward compatibility with 5.x blendfiles, and data
+   * management consistency.
+   */
+  struct IDProperty *system_properties;
 
   struct FreestyleConfig freestyle_config;
   struct ViewLayerEEVEE eevee;
diff --git a/source/blender/makesdna/DNA_sequence_types.h b/source/blender/makesdna/DNA_sequence_types.h
index 3c27617d3f1..aaaf599ed0f 100644
--- a/source/blender/makesdna/DNA_sequence_types.h
+++ b/source/blender/makesdna/DNA_sequence_types.h
@@ -296,6 +296,13 @@ typedef struct Strip {
   struct Stereo3dFormat *stereo3d_format;
 
   struct IDProperty *prop;
+  /**
+   * System-defined custom properties storage.
+   *
+   * In Blender 4.5, only used to ensure forward compatibility with 5.x blendfiles, and data
+   * management consistency.
+   */
+  struct IDProperty *system_properties;
 
   /* modifiers */
   ListBase modifiers;
@@ -305,7 +312,6 @@ typedef struct Strip {
   float speed_factor;
 
   struct SeqRetimingKey *retiming_keys;
-  void *_pad5;
   int retiming_keys_num;
   char _pad6[4];
 
diff --git a/source/blender/sequencer/SEQ_iterator.hh b/source/blender/sequencer/SEQ_iterator.hh
index 8c0421ca23b..38b11cdb3ea 100644
--- a/source/blender/sequencer/SEQ_iterator.hh
+++ b/source/blender/sequencer/SEQ_iterator.hh
@@ -8,6 +8,7 @@
  * \ingroup sequencer
  */
 
+#include "BLI_function_ref.hh"
 #include "BLI_vector_set.hh"
 
 struct ListBase;
@@ -32,6 +33,9 @@ using ForEachFunc = bool (*)(Strip *strip, void *user_data);
  */
 void for_each_callback(ListBase *seqbase, ForEachFunc callback, void *user_data);
 
+/** Same as above, but using a more modern FunctionRef as callback. */
+void for_each_callback(ListBase *seqbase, blender::FunctionRef<bool(Strip *)> callback);
+
 /**
  * Expand set by running `strip_query_func()` for each strip, which will be used as reference.
  * Results of these queries will be merged into provided collection.
diff --git a/source/blender/sequencer/intern/iterator.cc b/source/blender/sequencer/intern/iterator.cc
index 6c1f16c16c5..cadbb17c759 100644
--- a/source/blender/sequencer/intern/iterator.cc
+++ b/source/blender/sequencer/intern/iterator.cc
@@ -39,11 +39,33 @@ static bool strip_for_each_recursive(ListBase *seqbase, ForEachFunc callback, vo
   return true;
 }
 
+static bool strip_for_each_recursive(ListBase *seqbase,
+                                     blender::FunctionRef<bool(Strip *)> callback)
+{
+  LISTBASE_FOREACH (Strip *, strip, seqbase) {
+    if (!callback(strip)) {
+      /* Callback signaled stop, return. */
+      return false;
+    }
+    if (strip->type == STRIP_TYPE_META) {
+      if (!strip_for_each_recursive(&strip->seqbase, callback)) {
+        return false;
+      }
+    }
+  }
+  return true;
+}
+
 void for_each_callback(ListBase *seqbase, ForEachFunc callback, void *user_data)
 {
   strip_for_each_recursive(seqbase, callback, user_data);
 }
 
+void for_each_callback(ListBase *seqbase, blender::FunctionRef<bool(Strip *)> callback)
+{
+  strip_for_each_recursive(seqbase, callback);
+}
+
 VectorSet<Strip *> query_by_reference(Strip *strip_reference,
                                       const Scene *scene,
                                       ListBase *seqbase,
diff --git a/source/blender/sequencer/intern/sequencer.cc b/source/blender/sequencer/intern/sequencer.cc
index 50e27fcf87a..43e3512295d 100644
--- a/source/blender/sequencer/intern/sequencer.cc
+++ b/source/blender/sequencer/intern/sequencer.cc
@@ -214,6 +214,10 @@ static void seq_strip_free_ex(Scene *scene,
     IDP_FreePropertyContent_ex(strip->prop, do_id_user);
     MEM_freeN(strip->prop);
   }
+  if (strip->system_properties) {
+    IDP_FreePropertyContent_ex(strip->system_properties, do_id_user);
+    MEM_freeN(strip->system_properties);
+  }
 
   /* free modifiers */
   modifier_clear(strip);
@@ -534,6 +538,9 @@ static Strip *strip_duplicate(const Scene *scene_src,
   if (strip->prop) {
     strip_new->prop = IDP_CopyProperty_ex(strip->prop, flag);
   }
+  if (strip->system_properties) {
+    strip_new->system_properties = IDP_CopyProperty_ex(strip->system_properties, flag);
+  }
 
   if (strip_new->modifiers.first) {
     BLI_listbase_clear(&strip_new->modifiers);
@@ -791,6 +798,8 @@ static bool strip_write_data_cb(Strip *strip, void *userdata)
   if (strip->prop) {
     IDP_BlendWrite(writer, strip->prop);
   }
+  /* Never write system_properties in Blender 4.5, will be reset to `nullptr` by reading code (by
+   * the matching call to #BLO_read_struct). */
 
   modifier_blend_write(writer, &strip->modifiers);
 
@@ -879,6 +888,8 @@ static bool strip_read_data_cb(Strip *strip, void *user_data)
 
   BLO_read_struct(reader, IDProperty, &strip->prop);
   IDP_BlendDataRead(reader, &strip->prop);
+  BLO_read_struct(reader, IDProperty, &strip->system_properties);
+  IDP_BlendDataRead(reader, &strip->system_properties);
 
   BLO_read_struct(reader, StripData, &strip->data);
   if (strip->data && strip->data->done == 0) {