"You are licensed to use compiled versions of the Mattermost platform produced by Mattermost, Inc. under an MIT LICENSE[...]You may be licensed to use source code to create compiled versions not produced by Mattermost, Inc. in one of two ways[...]1. Under the Free Software Foundation’s GNU AGPL v3.0, subject to the exceptions outlined in this policy"https://github.com/mattermost/mattermost/blob/master/LICENSE...
This means that you can totally fork and patch any restrictions out of the publicly available source code, and there's not a bloody thing they can do to stop it.
Also, from a legal standpoint the "subject to the exceptions" part is bogus, because it is AGPL -- no exceptions can be made to that license, else it would not be copyleft anymore.