My best-guess is that the JID (Jabber Identifier) used for login is usually a clearnet domain to permit federation between XMPP servers without Onion Services, but it’s just a guess; I haven’t found anything that says this.
It seems like there would be a niche use-case for some folks who would be fine with just having Tor ↔ Tor communications. It greatly reduces the number of people you can message, but it greatly increases the privacy of your communications, and reduces the risk of metadata leakage, deanonymization attacks, correlation attacks, MITM attacks, etc.
Might be useful to post feature requests against servers that seem interested in supporting privacy.
I’m struggling to even find a list of XMPP servers with Onion Services
I found this, but only a few of them appear to be online. Unless these .onions only run XMPP servers (and they have no HTTP web server listening on port 80…)
Part of the problem is that I can’t find much info about how to use XMPP with Onion Services on the 'net.