There are multiple #Matrix
clients under development. I don't know whether they use the outdated proposal or just do what Riot does.
I don't know why so many pro-Matrix people are anti-XMPP and against every other attempt to build federated communications. With their emphasis on bridges to other services, they should be saying "sure, we can co-exist and interoperate with !XMPP
and whatever else, as long as bridges are allowed".
Work together to build something better than the #corpocentric
solution of the day instead of squabbling over which one is right or better.