I didn't review the code but this seems like a good way to fix bpo-46006. We can have more time to figure out how to make them per-interpreter (assuming that's the decision).

I agree with @nascheme that this is probably the best fix for 3.11a4.
I think longer term we will have to go for statically allocated identifiers.
Any other approach is just too error prone.

Let's hope that we can keep the overhead of immortal objects low enough that we can have parallel sub-interpreters and static global objects.

https://bugs.python.org/issue46006 is about interned strings. Fixing this issue doesn't need to share again identifiers between all interpreters. I'm not convinced by the value of moving the structures from PyInterpreterState to _PyRuntimeState.

I propose a different PR which only reverts ea25180 : PR #30422. It only moves the interned variable from PyInterpreterState to unicodeobject.c (static variable). Identifiers are left unchanged.

I merged my GH-20085 revert.