SHOULD generally means: some people might require it. implement it for best results

backward compatibility makes it hard to add MUST. using SHOULD is a good alternative

"SHOULD generally means: some people might require it."

No it absolutely does not mean that. It means, by explicit definition which is right here, that text is exactly that definition, that no one requires it. They can't require it, and still be conforming to the spec or rfc. That's the entire point of that text is to define that and remove all ambiguity about it.

It's not required by anyone.

The reason it's there at all, and has "should" is that it's useful and helpful and good to include, all else being equal.

But by the very definition itself, no people require it. No people are allowed to require it.

Any that do, are simply violating the spec.