So there are two approaches I see to solving this whole thing. One is for there to be a slight divergence between the C code and the Python code. For _warnings.warn(), nothing changes. For warnings.warn(), though, it does the expected frame skipping. This would mean that _warnings.warn() continues to exist for startup purposes but then user-level code and non-startup code uses the Python version that has the expected semantics. This requires figuring out why not importing _warnings.warn in warnings.py causes test failures.

The other option is someone helps me figure out why the C code is causing its test failures by triggering a -11 return in threaded/subprocess tests and prevent the divergence.

I have opened issue #24938 to actually re-evaluate one of the key points of _warnings.c which was startup performance. This was done about 7 years ago which pre-dates freezing code and providing simple C wrappers as necessary for internal use in Python like we do with importlib.