Huh, incidentally the typing analog of this sentinel seems to be dead code now: https://github.com/python/cpython/blob/96b7a2eba423b42320f15fd4974740e3e930bb8b/Lib/typing.py#L456-L459

It's last use (and it's global instance) were removed in python/cpython#136332

Opened python/cpython#138120

Let's merge this after the non-RC release goes out tomorrow.

Hmm, would we revert if PEP 661 was rejected?

If so, we'd likely keep Sentinel around in typing-extensions indefinitely for compatibility, and there's no real compatibility problem with changing this internal sentinel back to an internal class.