> I assume these breaking changes are in the stdlib and not in the python interpreter (the language), right?

That's the usual case, but it can definitely also happen because of the language - see https://stackoverflow.com/questions/51337939 .

> There was previous discussions about uncoupling the stdlib (python libraries) from the release and have them being released independently, but I can’t remember why that died off

This sort of thing is mainly a social problem.