You could use mypy in pre-commit so code is always checked. Sad there's no per-file switch to tell the interpreter 'this has to be checked'.
And yea, from an IDE perspective, it seems like maybe a sensible default would be to run `mypy ${CURRENT_FILE}` on save or something -- I've tried this manually myself, and it's decent, but it's not good enough to use constantly. I don't remember specific issues since I haven't done this in a long time.
Pre-commits are painful (on purpose!), but preventing people from shipping seemed like the wrong call. So we ended up dropping them.
Many people, including myself, also like making atomic commits that may not be fully typed yet. Conditioning people to skip the hook(s), doubly so during a gradual typing rollout, just conditions them to skip them always. Leave it in CI where it's checked at a point in time where it actually matters.