Beeper, universal messaging app that lets you chat with anyone on any chat app:

Beeper, universal messaging app that lets you chat with anyone on any chat app:
* https://www.beeper.com
* https://github.com/beeper
* https://beeperstatus.com

Apparently the code for the backend infrastructure is mostly if not all available, apart from the bridges they use which are already from the open source community. Newer mobile client is proprietary apparently.