Cannot add accounts to messages on Mac on my MacBook Pro Mid 2015 Version 10.14.6 I cannot add any other account (Jabber, G-Chat/Hangouts etc,) to messages. 

Can you still add third party accounts to messages? 

I tried to add it over online accounts (no option to tick messages) and I tried to add it in "message".
If you look at Apple’s support article about the messages app, nowhere does it mention being able to use third-party messaging services. Whereas in 2014, according to web.archive.org, the article said 3rd party messaging platforms where supported. It appears Apple has removed this feature.