Earlier quoted context omitted.
You'll notice that the google links mention that the xmpp api is deprecated. I'm not sure what Riot uses these days but the xmpp connection is no longer available to third parties. That sad truth is that people who actually use it end up dropping it. Its not some conspiracy. It just wasn't cutting it.
It's not XMPP that's deprecated, it's the whole CCS (first link) that's being replaced with Firebase (second link). The XMPP api is still present in the replacement.
While not fully consistent the XMPP api is referred to as legacy on much of FCM documentation.
https://firebase.google.com/docs/cloud-messaging/server
You also misunderstand what this api is referring to. These are APIs for your server to connect to Google's. This is not a direct http and xmpp connection to mobile devices. That's all abstracted away from this.