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Good Foss Alternative to Slack?
(beehaw.org)
If it's free and open source and it's also software, it can be discussed here. Subcommunity of Technology.
This community's icon was made by Aaron Schneider, under the CC-BY-NC-SA 4.0 license.
Matrix and Element work. It has apps for desktop IOS and Android. Can have multiple channels for different topics. Not quite slack level of workspace management, but it's an option.