It is de-facto required and has been for many years.

Should in most RFCs also mean "do it as long as you don't have a very good technical reason not to do it". Like it's most times a "weak must". And in that case the only reason it isn't must is for backward compatibility with older mail system not used for sending automated mails.

And it is documented if you read any larger mail providers docs about "what to do that my automated mails don't get misclassified as spam". And spam rejection is a whole additional non-standardized layer on top of RFCs anyone working with mail should be aware of. In any decades old non centralized communication system without ever green standards having other "industry standard/de-factor" but not "standardized" requirements is pretty normal btw.