That was the point of my opt-in suggestion (set schema_notifications=true). Then you aren’t changing anything for clients that work currently.
Alternatively you could wrap it all into a new protocol and just use a “set protocol_version=4”, or even better, push the protocol version in the startup parameters.
Well we would be adding messages that current protocol handlers may not know how to deal with which will likely require them to rewrite their code. That being said it would be easier to define a new protocol, even if it is just an extension and then older clients can still deal with the current protocol, and newer clients can request the newer protocol.