__subclasses__ is, as Tim Peters put it [0], "just Python exposing an
internal mechanism for the morbidly curious".
I'd expect that not all alternative implementations of Python have something like __subclasses__ -- it's a detail of the implementation (as opposed to the MRO, which is something any implementation of the Python language needs to be able to compute).
It certainly is cool functionality, but keep in mind we'd have to support it for decades, and it would put pressure on other Python implementations to add something like __subclasses__.

With that in mind, and because there's no real use case, I think it's better to not add this to inspect.

[0] https://mail.python.org/pipermail/python-list/2003-August/210297.html