Mattermost is a Free Software alternative to Slack.
The Slack client (desktop, web, and mobile) is nonfree software.
Slack centrally stores all postings permanently; the users have no choice.
Slack told a nonprofit that it would have to pay 50k dollars in a hurry or all its past discussions would be deleted. It is ironic to read the discussion which slammed the company for this, but failed to recognize that making their participants use nonfree software to join in their discussions was itself a form of mistreatment.
The Slack's API TOS forbid replicating the product or service using the API would would seem to prohibit making a free software client for Slack.
Even when connecting with an alternative client, using the Slack website (with its non-free javascript) is required to make an account.
The Slack's API TOS can be changed arbitrarily by Slack.
The Slack's API TOS prohibits deriving 'know-how' which would seem to prevent a free software developer who agrees to the TOS from working on future Slack competitors.
Slack can and does determine the user's geolocation, and uses that to block connection from Iran and some other places where the US has imposed sanctions.
Slack initially provided gateways to allow connecting from IRC or XMPP. Eventually Slack turned these gateways off, providing no ways for users who were accustomed to using standard protocols and Free Software to connect any longer.
There is no offline or local archive of Slack messages and no way to retrieve old messages if your account is cut off.
By default Slack fetches a copy of and scans all URLs you include in your messages.
Slack's (changeable) privacy policy does little to prevent data mining by the company or any future purchaser of it.
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Copyright (C) 2019, 2020, 2021, 2022, 2023, 2024, 2025 Richard Stallman
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Research sourced from stallman.org.