> The ABI is not broken, the only thing that this PR change is the size of the struct. All the offsets to the members are the same and therefore will be valid in any compiled code.

I'll just note that a change in struct size does technically break ABI, since *arrays* of PyThreadState will break.

So the size shouldn't be changed in RCs or point releases. (But since it's not part of stable ABI, it was OK to change it for 3.10.)

> We will try to determine and open a discussion in the future about what is considered public/private in these ambiguous cases and what can users expect regarding stability and backwards compatibility.

Please keep me in the loop; I'm working on summarizing my understanding of this (in a form that can be added to the docs if approved).