I can confirm that's not the case. We have diagnostic and error reporting functionality built-in which is documented here https://docs.mattermost.com/manage/telemetry.html and can be disabled either through the config file before starting the server or in the UI afterwards.

We have quite a few customers deploying Mattermost in air-gapped environments, so even for the enterprise versions we don't require internet connectivity.

There are some things that may not work without any internet connectivity, such as mobile push notifications, since that requires Mattermost Server to connect to our Push Proxy by default. You can host that yourself but will need a custom Apple/Android mobile app then, and the push proxy still needs internet connectivity in the end to reach Apple/Android Push Notification Services. Also our plugin marketplace may not work and you need to download and upload plugins manually.

So overall, some convenience functions may not work, but overall Mattermost can be deployed air-gapped and doesn't have any phone-home aside from optional diagnostics. Let me know if there are any other questions!

(Disclosure: I am working at Mattermost in Security)