It wouldn't be the first time the specs have gone too far and beyond their perimeter.

C's "register" variables used to have the same issue, and even "inline" has been downgraded to a mere hint for the compiler (which can ignore it and still be a C compiler).

inline and register still have semantic requirements that are not just hints. Taking the address of a register variable is illegal, and inline allows a function to be defined in multiple .c files without errors.