I have a problem with this approach when I transfer two or more animation folders into the project. OnPostprocessAllAssets catches only the last created _SkeletonData.asset. The rest of the created _SkeletonData.asset files do not trigger this callback.
Unfortunately I could not reproduce this behaviour on 2023.2.16 with a separate class and the OnPostprocessAllAssets
callback, I received a callback for each _SkeletonData.asset
file with none missing.
Since the workaround on the Unity forum here is neither clear (shall we write unique files to the Artifacts dir, and then what next?) nor clean, we will wait until Unity provides a proper solution, and until this becomes a real problem.
Anyway, thanks again for sharing your findings!