Another problem is what happens when an implementation uses non-"virtual" inheritance:

>>> issubclass(collections.UserDict, collections.abc.Mapping)
True
>>> collections.UserDict in collections.abc.Mapping.get_virtual_subclasses(True)
False

IOW, I think this get_virtual_subclasses() proposal is too low-level to be commonly useful, since it depends on implementation details (whether subclassing is direct or not, whether registering is implicit or explicit).