Yes, XMPP, a long-standing protocol that’s also not a walled garden, doesn’t require a phone number or even a phone. For android I use the Conversations client combined with Dino on computers. Currently logged in to a handful of devices synchronously. You can choose what server to make an account on; conversations.im I found to be reliable. Drawback is Signal doesn’t let you bridge to it from anywhere outside of Signal. So I have accounts on both.
You still need a phone number to register an account as far as I could tell when I did the other day. You no longer need to share your number with any contacts and can set it so noone who has your number can look you up on signal. You can optionally set a unique alphanumeric ‘username’ instead to hand to people to look you up. But yea, Signal still requires you to give them and their authenticatian service (through sms code) your phone number.
Well if only those samsung & ios users that never get my messages until I see them and tell them to open their app had phones that didn’t interfere with it running in the background / push notifications it would be working out for me even better, but that’s not an issue with the protocol or client but with OS’s being hostile to xmpp.
I use xmpp all the time. Biggest hurdle for certain fam/friends using xmpp has been certain android builds (samsung) and ios interfering with timely notifications. User knowlege is not a problem as I can recommend the apps that are compatible encryption protocols with mine.