For printf(), that would work. For sscanf(), POSIX could add macros there, too, but sscanf() does not detect integer overflow. That’s why the only previous solution was strtoimax() which does detect overflow.

scanf not detecting integer overflow is arguably another problem. It's usually the easiest way to parse numbers out of simple strings, but the lack of overflow detection is a problem. (Actually we use sscanf for this and ignore the overflow problem, as long as it doesn't cause any security or correctness concern.) IIRC glibc actually does overflow detection, so there's not anything inherently problematic with adding it.