bpo-46355: What's New: Note that PyFrameObject are private by encukou · Pull Request #31032 · python/cpython

the-knights-who-say-ni

Feel free to send a similar PR.
I like to use similar wording as the docs, but I also check the docs of several past versions, to be fair to people that wrote their code long ago.
So for frame I added the "subject to change at any time", but threadstate docs say the members are not "public", so just calling them "private" should work.