Google stopped supporting XMPP in the sense that they stopped trying to change XMPP to fit their needs. They also started killing off support for arbitrary XMPP servers, e.g. you cannot run your own and expect it to work with Google Chat. (This was the most up to date link I could find.)[https://xmpp.org/2015/03/no-its-not-the-end-of-xmpp-for-goog...]
You can still somewhat use it, but it does not work well. Groupchats do not work at all anymore, and there is no message synchronization over it so any Hangouts client will populate messages an XMPP client never sees, and you cannot request chat history from the remote.
It is definitely not a high quality XMPP experience. If you want to use XMPP, you should probably be using duckduckgo's XMPP offering.