Summary
Beeper is basically all of your chats in one app. It is built on Matrix, an open source federated messaging protocol. Among the messaging services are WhatsApp, Telegram, Signal, Instagram and Twitter DMs, Messenger, Skype, Hangouts and others.
Quotes
My thoughts
With the mess of each messaging APIs I thought we'll never see a modern version Trillion. Anyone who has dealt with APIs from social networks will know the headache. However, it is understandable that API calls are limited as it can be used for spam. This is were Matrix comes in with it "bridging" function (a relay that connects other messaging services to Matrix). The matrix team will maintain these bridges and developers only need to deal with a single API from Matrix.
Sources
https://techcrunch.com/2021/01/21/pebble-founder-launches-beeper-a-universal-chat-app-that-works-with-imessage-and-others/