This is a periodic reminder that Facebook never federated with XMPP and thus never turned it off. This is a pervasive myth. (Google did federate and eventually killed the whole product that federated, but Facebook never federated atall with XMPP.)

There is a hierarchical version of xmpp: URIs but it is rarely used since that would denote a server to connect to and usually you just want to mention an address (ala mailto: or tel: or sms: URIs)

check out eg https://wiki.xmpp.org/web/SRV_Records for how to delegate your name to a server and https://kingant.net/check_xmpp_dns/ for a tool that checks you did it right

Working on it! Doing alpha testing now but I hope for something more polished within a year.

You can move you account to another server, I do it all the time! It uses the same technology as ATProto names, where DNS can delegate what server to use for the name