backward compatibility makes it hard to add MUST. using SHOULD is a good alternative
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.
SHOULD means that if you don't that, bad things are likely to happen, but it will not immediately break at the protocol level and during discussion in the IETF some people thought there could be valid reasons to violate the SHOULD.
Typically, IETF standards track RFCs consider the immediate effects of the protocol but often do not consider operational reality very well.
Sometimes operational reality is that a MUST gets violated because the standard is just wrong. Sometimes a SHOULD becomes required, etc.
Certainly for email, there is a lot you have to do to get your email accepted that is not spelled out in the RFCs.
Should just means the thing is preferred. It's something that is good and useful and helpful to do.
That is not "must unless you can convince me that you should be excused".
For most cases you should use three points of contact. However, there may be other situations for example if someone is giving you a leg up, or you can pole vault, where another solution is preferred.