Well, there are plenty of open solutions that you can self host. Personally, i find Rocket.Chat ( https://rocket.chat/ https://rocket.chat/ ) to be simple enou

Well, there are plenty of open solutions that you can self host. Personally, i find Rocket.Chat ( https://rocket.chat/ https://rocket.chat/ ) to be simple enough to set up in a Docker container and have running as a passable alternative to Slack/Discord. There are downloadable clients as well. The only real downside I've seen is somewhat bad support for calls because their WebRTC implementation is still new and integrating with Big blue button or whatever else they offered is hard.