Slack is one of the most popular team communication services for work. Some may call it a glorified IRC but that doesn’t impact its popularity. It is available for free with additional features offered in its paid plans. Though Slack can be installed on Linux thanks to an Electron app but it is not open source, neither the client nor the server.
A further issue with Slack is that you cannot install and manage it on your premises so your data sits in another country.
The linked article runs through the acknowledged best free alternatives such as Riot, Rocket Chat, Zulip Chat, Wire and Mattermost.
See Top 5 Open Source Slack Alternatives for Team Communication
Looking for an open source alternative to Slack? Here are some of the best open source team communication software you can try.