From the README:
Mastodon 4.x radically changed the frontend, to much dismay from the actual community. It’s now a (slow) webapp, which requires access to lots of API routes that were previously unavailable to unauthenticated parties. It gives the public a much deeper view into your (private) community, both non-techincal (instance home pages now show an ‘explore’ page nobody asked for, that shows public content from instances you federate with. [you have to fully disable trending]), and on a technical level (toots and search API are publicly available allowing for much easier programmatic scraping).
XMPP actually has that right now, as you can restrict publish-subscribe nodes to contacts. Which is different from private groupchats in that you have unified timeline interface rather than separate chats in the few clients that support it right now.
Personally I strongly dislike this context-less mode of communication and very much prefer topical chatrooms and fora, but to each their own. I just wanted to note this exists and encourage people to try Movim and/or Libveria (both are web based) if that is something people are interested in.
Libervia recently also added experimental OpenPGP e2ee encryption to their pubsub implementation.