I think the problem is that the license grant (that has been in place for a decade) is not that clear.
You are licensed to use compiled versions of the Mattermost platform produced by Mattermost, Inc. under an MIT LICENSE
- See MIT-COMPILED-LICENSE.md included in compiled versions for details
You may be licensed to use source code to create compiled versions not produced by Mattermost, Inc. in one of two ways:
- Under the Free Software Foundation’s GNU AGPL v3.0, subject to the exceptions outlined in this policy; or [...]
I read it as releasing the binaries under MIT and granting people an AGPL license for the (non-enterprise) code. Some read it as not granting you the full AGPL rights.
To me, the fact that they advertise Mattermost as "open-source" and the statement on the "reciprocal license" above indicates that Mattermost also reads this as an AGPL license grant. However, they don't seem to be interested in fully clarifying the license situation. But, I think they would have a very hard time to argue in court that this license doesn't allow AGPL forks. And I haven't seen any evidence of them acting against any of the existing forks.