Today is Jabber's 20th anniversary!

On this day in 1999 Jeremie Miller announced an "Open Real Time Messaging System" on Slashdot.

https://t.co/EMfK3a7QSv

Can anyone tell how to use it?

Sure. Get an account (you may choose any server, here is a list of some https://list.jabber.at/). Log into any Jabber supporting app (e.g. Conversations on Android, Gajim on Desktop).

Share your jabber Id with your contacts and start chatting, you don't have to have accounts on the same server.

That should it be.

I genuinely think the only place I've seen it used in recent times was for EVEO