Per @AlexWaygood:

All the ABCs in collections.abc deliberately define __slots__ for this very reason: so that other classes can subclass those ABCs and still be able to use the __slots__ machinery to block the creation of instance dictionaries.

Linked PRs

• GH-104898: Add __slots__ to os.PathLike #104899
• GH-104898: Revert pathlib os.PathLike registration change. #105073