I'll defer to typing gurus as to whether something like this might make sense in typing.py, but it doesn't make much sense as a collections.abc without an API exposed in pure python and without useful mixin methods.  By way of comparison, consider that the named tuple protocol is in typing.NamedTuple but  not in collections.abc; instead, we just use _fields to recognize it when the need arises (which is almost never).   

I would like collections.abc to remain a place for ABCs that can be usefully subclassed, that provide guaranteed behaviors, and that are in fact related to collections.  The module's utility will be impaired if it becomes a dumping ground for miscellaneous type hinting concepts and registrations.

Early on there was some experimentation with the "one-trick ponies" in collections.abc such as Callable and Hashable that turned-out to almost never be used in practice, so we don't want to continue to that line of development when there are more promising avenues such as typing.py.