@debacle jabber.org is indeed the oldest running XMPP server. It was originally the dev server for the Jabber project, years before "XMPP".
It grew into a public service, until registration closed in (I think?) 2013.
Don't expect a sudden change in features after the migration. It's already going to be quite a challenge, so we decided initially to keep close to the current feature set initially.
Then when we are happy it's stable, gradually enable all the new modern stuff we've been lacking 🙂