If you were online 7/21/17 you should have received an announcement via XMPP about the new domains that were being added.
As of a couple days ago I’m happy to say they’ve been setup for public use and you can register an account on any of them you’d like. If someone has taken a cool username on say, @xmpp.is you could just register it on @xmpp.co, @xmpp.cx & @xmpp.xyz vice versa. All of the domains support DNSSEC and the same security practices have been applied (no logs kept, full disk encryption, PFS supported & TLS required). All requests go to the same server specifically so communicating between these domains is even more secure. If you’re interested in grabbing an account go to the register page!
I’d also like to mention that the site and XMPP server (Prosody) now entirely use Let’s Encrypt. In the past I wasn’t able to automate the renewal of certificates because Prosody would have to be restarted every time the cert was renewed (every 3 months). To minimize downtime I found that you could just reload the Prosody TLS module when reloading the service using mod_reload_modules. This seemed to work flawlessly in tests. Along with this, a systemd service for renewals and my new script, all certs are now provided by Let’s Encrypt!