I don't have this experience. Can you give an example of a previously-valid codebase that failed typechecking unexpectedly on a recent MyPy update, that wasn't a result of a false negative bug in MyPy?
I can't give you any links because it's not open source code, but there was a bug fix in 0.790 named "Don't simplify away Any when joining union types" that caused me some problems with bad annotations in the existing code: The annotations implied that Any was possible, but it wasn't, but it got dropped from the final Union by the bug so we never had to handle Any. Dataclasses have had some backwards-incompatible improvements as well.
But the big culprit is typeshed. Something will get new/fixed annotations and suddenly you aren't handling all possible return types in your callers, or whatever.
I've had this problem a few times., For example it happened with the 0.800+ versions. Mypy got stricter, and was more aggressive in finding code (eg small scripts in not in the proper python hierarchy).
It's hard to classify anything as a "false negative" with mypy since it is very liberal (often unexpectedly so, which I think is one of the sharp edges of gradual typing).
I obviously can't share it here, but my primary codebase at my job needs a few dozen changes every time we change mypy versions. There's a few places of false positives where I've had to comment `# shut up, mypy` and a `type: ignore`. Usually when I'm being clever with the subprocess module.
I wonder if that's a mypy issue or more that the typeshed types are bugged, since type shed versions also get shipped (used to?) with new type checker versions.
