One cannot not say something is "so open that..." ;-)

But indeed, XMPP is open, very open.

And indeed the major clients usually rely on the old era's ways of thinking messaging... They a far from contemporary, not only talking of features, but also the UX (User eXperience) and the general philosophy.

I would recommend the reading of XEP-0387: XMPP Compliance Suites 2017 (and its successors): https://xmpp.org/extensions/xep-0387.html

But I insist, this one is quite good, honestly: https://www.erlang-solutions.com/blog/21-xmpp-use-cases-and-...