TFA mentions that Mattermost's source is AGPL and the binaries are MIT. It's absolutely free software.

It's also $0 software, in that it being free software and open source, you can simply and legally patch out their license checks and recompile, if you so wish.

How is slack any better than discord? Both roll out stupid (and forced) updates, and slack doesn't even let me browse kinda old history if the owner doesn't pay for it. [*4] Not to mention the search is relatively terrible anyway.

The main thing that worries me about is discord is their revenue model... I just don't see how they're making money, and they keep on raising money.

There are a few good alternatives like zulip [0], matrix [1], discourse [2], Rocket Chat [3], or just plain old irc.

Although discourse isn't really a chat application but more of a open source forum software.

[0]: https://github.com/zulip

[1]: https://www.matrix.org/

[2]: https://github.com/discourse/discourse

[3]: https://rocket.chat/

[4]: You can get around this by messaging relevant information to yourself. Or just saving things locally, like a weirdo.