How is #XMPP (a.k.a. #Jabber) not decentralised? It is as decentralised as the #fediverse in my book.
As with the fediverse, you can choose your server. E.g. #disroot in NL, xmpp.is in IS, #conversations in DE, etc.
Find more servers here:
https://joinjabber.org/docs/servers/ by @joinjabber
or here:
https://providers.xmpp.net/ by @xmpp_providers
You can also selfhost, made esp. easy with #snikket by @snikket_im.
-list, and the free, federated services in the second 
-list 