IME ObjC (unlike C++) automatically supports the latest C standards, because ObjC just adds some syntax sugar on top of C, but otherwise doesn't 'mangle' its C subset into its own dialect like C++ does.
This is only true as long as the changes in C aren't conflicting with ObjC. In the future if ObjC is deemed dead by the council it won't have to worry about such things. So C58 may have tons of things that don't work in ObjC98 (or whatever the ObjC version is).