I think subclassing builtin types and making it abstract is rare. And when there is a need, we can mimic this in application level (this may also apply to types having custom __new__):

In [2]: class CustomDict(dict, metaclass=abc.ABCMeta):
   ...:     def __new__(cls, *args, **kwargs):
   ...:         if getattr(cls, '__abstractmethods__', None):
   ...:             raise TypeError
   ...:         return super().__new__(cls, *args, **kwargs)
   ...:     @abc.abstractmethod
   ...:     def f(self):
   ...:         pass

Adding the abstract class checking in tp_alloc or builtin types' tp_new maybe degrade performance.

Is it necessary to add this support?