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It's more a Slack alternative than a Trello alternative ? Their website is so bad that I had to go to wikipedia to remember what mattermost is.

You're right, Slack/Trello alt is what I view it as, but the Trello part was more relevant for me.

For anyone reading, Zulip (for Slack) and Wekan (for Trello) are good replacements.

I needed to switch an org away from Slack, seems like Zulip has a new customer :).