Mattermost began as a free & open source self-hostable Slack clone. I deployed it in 2018 on a server and have used it every day since, in support of at-risk environmental defenders. Today it hosts a very large community.
In this time Mattermost has come to be far superior to Slack (which I regretfully have to use weekly), in almost every capacity.
Yet another example where the narrative that truly free software will never eclipse the feature set or productivity of proprietary, is challenged