I skimmed the standard and POSIX, but couldn’t find anything that an implementation is required to support FENV_ACCESS.

https://en.cppreference.com/cpp/preprocessor/impl:

“ISO C language standard requires that C compilers support the following three pragmas, and some C++ compiler vendors support them, to varying degrees, in their C++ frontends:

  #pragma STDC FENV_ACCESS arg      (1)
  #pragma STDC FP_CONTRACT arg      (2)
  #pragma STDC CX_LIMITED_RANGE arg (3)

⇒ it seems C compilers must support this (but C++ compilers need not)