This project is certainly aiming to push things forward. Indeed a decade ago it would be totally unsurprising to see a client with a drop-down for "STARTTLS", "

This project is certainly aiming to push things forward. Indeed a decade ago it would be totally unsurprising to see a client with a drop-down for "STARTTLS", "Direct TLS", "No TLS" (!). I think it's best described as something of an evolving "current best practices" document. There is certainly room to tell developers to implement (or not implement) certain features. Indeed that is done already for some things (e.g. file transfer mechanisms). I think this specific TLS example is unfortunate because it's something that is documented only for advanced use (recognising that some people may need it for specific purposes that some clients may want to support).