RocketChat/Rocket.Chat.RaspberryPi is an open source project licensed under MIT License which is an OSI approved license.
The ultimate Free Open Source Solution for team communications.
Zulip server and webapp - powerful open source team chat
Scout APM - Leading-edge performance monitoring starting at $39/month. Scout APM uses tracing logic that ties bottlenecks to source code so you know the exact line of code causing performance issues and can get back to building a great product faster.
Matrix (An open network for secure, decentralized communication) server setup using Ansible and Docker
whatsapp replacement, easy to set up, that does not not downscale photos and videos
reddit.com/r/selfhosted | 2021-04-08
If you only want the messaging feature then you could also try Rocket chat (run on a raspberry and the main repo) or Zulip. They are both open source and easy to self-host (I am not sure how easy it is to setup Zulip but it is quite popular).