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Note: I've covered case with __subclasscheck__ to ensure that behavior remains the same even if the internals of ABCMeta change. But it's mostly an implementation details, not a public contract. I haven't found documentation which covers or recommends using it.

I also haven't found any evidence that it is used in the wild, by using Github search. __subclasscheck__ are used in mocks, custom non-ABC metaclasses and in typeshed annotations:
https://github.com/search?q=%22def+__subclasses__%22+language%3APython&type=code&ref=advsearch&p=1
https://github.com/search?q=%22__subclasses__+%3D%22+language%3APython&type=code&ref=advsearch&p=1

Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.