The question of whether an implementation provides any useful diagnostics in any particular situation is a Quality of Implementation issue outside the Standard's jurisdiction. If an implementation were to unconditionally output "Warning: this program does not output any useful diagnostics" or even "Warning: water is wet", such output would fully satisfy all of the Standard's requirements with regard to diagnostics even if the implementation didn't output any other diagnostics.
Further, the authors of the Standard characterized as "Undefined Behavior" many actions which they expected would be processed in a meaningful and useful fashion by many if not most implementations. According to the published Rationale document, Undefined Behavior among other things "identifies areas of conforming language extension", since implementations are allowed to specify how they will behave in cases that are not defined by the Standard.
Having implementations issue warnings about constructs which were non-portable, but which they would process in a useful fashion would have been annoying.
Prior to the Standard, some implementations would usefully accept constructs like:
struct foo {
int *p;
char pad [4-sizeof (int*)];
int q,r;
};
for all sizes of pointer up to four bytes (8-byte pointers weren't a thing back then), rather than squawking if pointers were exactly four bytes, but some people on the Committee were opposed to the idea of accepting declarations for zero-sized arrays. Thus, a compromise was reached where compilers would squawk about such things, programmers would ignore the useless warnings, and the useful constructs would remain usable on implementations that supported them.
While there was a vague attempt to distinguish between constructs that should produce warnings that programmers could ignore, versus constructs that might be used so much that warnings would be annoying, the fact that issuance of useful diagnostics was a Quality of Implementation issue outside the Standard's jurisdiction meant there was no real need to worry too much about such distinctions.
