gh-95913: Consolidate build requirements changes in 3.11 WhatsNew by CAM-Gerlach · Pull Request #98781 · python/cpython

Well, I chose to be extra explicit, since compilers have the bad habit of not implementing fully or exactly C standards. For example, see Modernizing Fedora's C code about GCC and clang which don't respect C99/C11 "for backward compatibility".

Yeah, but that (the example at least) is about keeping old behavior in cases where C99 says something is illegal or undefined. (AFAIK, the standard mandates an error, so the implementation doesn't comply with the standard, but it's fine in the real world.)
Leaving out <math.h> (or a function from it) would be pretty hard to sell as a deliberate backwards-compatibility quirk. So in this particular case, only requiring a C11 libc should be fine.