Many otherwise open-source chat apps are "open-core," they tie certain features to a subscription. Can be things like chat history, voice calls, video calls, but a very popular one is SSO and AD/LDAP integration.
replyYeah a lot of them like Mattermost become surprisingly limited unless you pay. It's very annoying.
replyMattermost's licensing is a little confusing, but from what I understand, you're only really super-restricted if you use the prebuilt binaries (which have a different license than the source code).
replyIIRC if you build it yourself it's pretty much all AGPL, with few limitations.