ABI is not "whatever happens to work with this distro" but "what programs that comply with the API contract compile to". Overlapping memcpy arguments is an API contract violation and thus not something covered by the ABI either. This distinction is the entire reason why C has a separate memmove function. You can't just make up your own imaginary ABI contract and then blame the system when it doesn't fulfill it. That's going to result in self-inflicted plain on any OS.

> It depends on how we define the ABI. I see it as a set of client-visible invariants that they rely on

By that definition, any change of any kind will break ABI, Hyrum’s law and spacebar heating and all that.