The current status of the PEP is that it's stalled due to me not finishing it. I got caught up in the typing discussion, largely due to not having much previous experience with Python typing, and failed to come to a clear conclusion of what a good solution would be.
I think you've been given all the options in the thread, and you just have to pick one. The best way to find out what mypy currently supports is to install it and try it out on some test files (make sure to run `mypy --strict`).
Indeed, that's my impression too.
After going through the discussions again, I'm going to go with using Literal[SentinelValue]. I'll add that to the PEP soon.