> Lets say four services inter-communicate with some protocol, it is extremely annoying to impose a deployment order where the producer of a message type is the last to upgrade the message schema

i don't know how you arrived at this conclusion

the protocol is the unifying substrate, it is the source of truth, the services are subservient to the protocol, it's not the other way around

also it's not just like each service has a single version, each instance of each service can have separate versions as well!

what you're describing as "annoying" is really just "reality", you can't hand-wave away the problems that reality presents