collections.ChainMap.__missing__ is defined in typeshed here:

But, at runtime, it always raises KeyError (and always has done), so there's no situation in which it would be safe to call this method (it's really just an implementation detail to give ChainMap a nicer error message for when the key isn't present in any of the underlying mappings, as far as I can tell).

The method was added to typeshed in a standalone PR (#4784)... Am I __missing__ something? Is there a good reason for it to be there?