> IMO this PEP is outdated for a long time.

It is not. Even if it is, it should be marked as such, and that is not a decision that should be done in this issue.

Please, don't break the rules because you think they're outdated.

> Well, if you ask me, I would simply require a C99 compiler. That's all :-)

Please propose that change. Perhaps it would be a good change to make, but I don't even know how to determine that.
Nor can I list the places where the change should be made -- at least there should be a What's New entry like https://docs.python.org/3.10/whatsnew/3.6.html#build-and-c-api-changes

> Note: Python uses C11 <stdatomic.h>, but it remains an optional requirement.

That's fine. You can still build with an older compiler.