For IDProperties, this flag is mostly used only to controll whether
IDProperty code should also handle ID refcounting or not (through
`LIB_ID_CREATE_NO_USER_REFCOUNT`).
Without this fix, IDProp Group Merge could end up trying to modify ID
refcount of its data-block properties (via `IDP_FreeProperty`), which
can lead to data corruption or even crashes (e.g. when used in readfile
code, before lib-linking process).
Issue discovered while working on the system IDProperties split project
(!135807).
Pull Request: https://projects.blender.org/blender/blender/pulls/139230
647927a330