Maybe the standards documents you are used to differ from RFCs, but here is the official language:

   3. SHOULD   This word, or the adjective "RECOMMENDED", mean that there
      may exist valid reasons in particular circumstances to ignore a
      particular item, but the full implications must be understood and
      carefully weighed before choosing a different course.

SHOULD is effectively REQUIRED unless it conflicts with another standards requirement or you have a very specific edge case.

Assuming we’re talking about RFC 2119, it’s important not to collapse the distinction between SHOULD and MAY, which is there for a reason. MAY elements are legitimately optional, SHOULD elements are there for a reason and are disregarded at one’s own risk.

> SHOULD This word, or the adjective "RECOMMENDED", mean that there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course.

To validly disregard a SHOULD, you need to (a) fully understand the implications, and (b) accept them.

Any time someone disregards a SHOULD and then complains about the result, they are necessarily in the wrong. Either they didn’t fully understand the implications, or they don’t actually accept them.

Agreed. We use RFC language at work for documents, and whenever I see someone using the word SHOULD, I always ask what is the MAY where you don't need to do the thing? If they can't think of one, then make it a MUST. SHOULD always means there are valid reasons to not do it.

FYI slightly over 10% of RFCs contain at least one occurrence of SHALL. Even that small minority uses other words for most requirements.

Email is about standards like browsers were about standards in 2017...

Could was yeah nice to have. Should, was yeah the system shall have it, but it's ok if not. Shall : ffs we will hunt you and kill you if you don't implement it.

Yes, except there seems to be a move on the best words from SHALL to MUST and from SHOULD to MAY. IANAL but I recall reading this in e.g. legal language guidance sites.