It's FOSS takes a look at Rocket.Chat, an open-source alternative to Slack that you can self-host.

Rocket.Chat: An Amazing Open-Source Alternative to Slack That You Can Self-host

Rocket.Chat is an open-source team communication application with features and looks similar to Slack. You are free to self-host it or opt for their managed service for a fee.

Rocket.Chat: An Amazing Open-Source Alternative to Slack That You Can Self-host - It's FOSS