Login with a Jabber/XMPP client (like http://pidgin.im ) to the Google account and chatting with Hangouts users by this is still possible (tested last night about 1:00 am UTC).

At the moment it looks like Google stopped the Jabber/XMPP federation.
That's the feature which makes it possible to chat between Jabber.org or GMX.net and many other XMPP chat servers like GMail. And this is the most important feature, because it used to make GoogleTalk different from chat networks like ICQ, MSN, Skype or WhatsApp. With the federation feature XMPP/Jabber becomes network like the email-"network" where everybody can choose his favorite provider and nevertheless communicate with everyone else at every other provider.

I'm actually collecting some information about this issue here:
https://productforums.google.com/for...I/Lb-avo0zPwYJ