Interesting. Notably, they also call themselves "open source" in the "About" of the repository. I'm not aware of any critical extensions which are closed-source. The change you've highlighted was made 4 months ago under a commit that gives no explanation: https://github.com/mattermost/mattermost/pull/31247/files , and the discussion there is private.

Notably, they do have some "source-available" code that goes into the enterprise release, at https://github.com/mattermost/mattermost/tree/master/server/...

This mainly seems to relate to metrics and fuzzy search, though it's possible more will move here in the future (it looks like this is a relatively recent development). Until recently they also had experimental support for Bleve full-text search (now seemingly deprecated), but the elasticsearch enterprise feature seems to be the replacement (otherwise they use postgres's ILIKE for built-in text search)

So, all told, Mattermost was open source, and may be moving to open core. Which means now is probably the best time to create a community-maintained fork. The team edition, and almost all features, are currently still open source.