I like it. I can't see how to do better while remaining compatible with Python idioms.

"A simple cache returns None both for a missing key and for a key that maps to None."

No, it returns KeyError. Or at least it should.

When I was writing a bunch of Lua, I was a big fan of having a None sentinel value (Lua uses 'nil' for representing absence) and it was useful for a lot of things similar to Pydantic's MISSING.

This distinction is something that I think is actually handy about JavaScript, and distinguishing between 'null' and 'undefined' is sometimes useful for similar reasons.