In the Action editor header, change the "Show All Slots" filter to
become "Only Show Slot of Active Object". The default state remains
"off".
This means that any new/upgraded Action editor will show all slots in
the Action by default. I think this is a good idea, especially since
cb6ed12ef1 makes related data-blocks share
the same Action, and thus the Action will have a mixture of Object and
non-Object animation. I suspect that it'll help in understanding the new
functionality of slotted Actions when they are most visible, i.e. when
the multiple slots are all shown.
Since slotted Actions are so new, I don't think we need to add
versioning code to accomodate this change.
Pull Request: https://projects.blender.org/blender/blender/pulls/129011
975b5f4533