Matrix is a new-ish decentralized, private, E2EE encryption protocol. It's pretty neat. It still has some issues (at least that I experience. Mainly the Android app is constantly being super slow to receive messages), but it's super promising.

They also have some goals to improve email infrastructure by integrating the matrix protocol, but not sure if that will go anywhere. I remember reading this off hand remark on their blog. Can't find the source.

As the original comment said, there's the concept ifa "bridge" which allows you to bridge other services to a matrix chat. So you could have a discord channel and matrix room bridged, as an example. A ready to go option with bridges is Beeper. But you can also setup your own stuff, as they said.