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.