You assume that internet standards are prescriptivist; that the document describes how it is to be implemented. In practice it's often descriptivist, with the standards documents playing catch-up with how things are actually going in practice.

Anyway, in general you can expect that doing unusual but technically valid things with email headers will very often get your messages rejected or filtered as spam.

Standards are definitely prescriptive. But just like a medical prescription, it doesn’t ensure that actors in the wild will conform to what’s prescribed. People will not follow prescriptions for whatever reason, willingly or otherwise. It doesn’t mean the document wasn’t prescriptive.