Not sure how popular the small federation was back then, but I know Mac OS X Server touted an XMPP server and that was a first-class feature of iChat.
Google Talk was real and federated XMPP before they killed the product.
I remember this, it was great to connect to absolutely every chat platform with bitlbee and pretend that all my chats were just DMs on some irc server somewhere
I haven’t had a reason to use an xmpp client in over a decade.