Maybe, but the services might also be allowed to offer completely custom APIs. The proposal isn't quite clear on that. Also, at this stage it's just a draft that has a long way to go until it can come into effect and the chances that it gets altered into something completely different along the way are still pretty high. Don't get your hopes up yet.

And/Or Matrix

It should be a rule that you have to have implemented a protocol before you can start demanding that others do so.

No demanding here, just wishful thinking. I am aware that the Matrix protocol is way more complex than other chat protocols.