Progress on no-GIL CPython

That is not a compatibility break. That is a quality of implementation issue. If the code still works and gives the right answer in a somewhat-believable* amount of time, then it is compatible. The question is whether this is a good or a bad tradeoff to make, not whether it will break anything. Offering two versions of the interpreter (or better yet, a flag/environment variable) would be one way to mitigate that issue, but they might instead reasonably decide that maintaining both versions is not worth the effort.

* I'm not going to precisely define "somewhat believable amount of time," but if performance on a modern system is comparable to the previous version on hardware released within the 21st century, then that's probably good enough. If it runs like a PDP-11, then that's probably too slow.