@raccoon well it's unfortunately not fully open source and does have some Big Tech stuff built in (Google Assistant, Maps)
There's a custom ROM called #GerdaOS specifically for the Nokia 8110 Bananaphone that is fully Free Software. There was also #ProjectPris for other phones but there's been no news on that for a while 🙁
Other two hurdles I think are app availability and sluggish hardware. As for #XMPP support...kind of, with #Convo, and I'm working on it 😉