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.
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.