Slack has raised our charges by $195k per year

Note that Mattermost is fake open source cosplay, and keeps important features in their non-foss application. If you want these table stakes features (like SSO or message expiry) you’ll find yourself maintaining your own fork or janky scaffolding (I have cronjobs that run SQL directly against the db).

They are using open source licenses simply as marketing for their proprietary enterprise software product.

It’s still better to self host than to use a SaaS, but the situation isn’t improved quite as much as one might think.

Mattermost is open source, but the licensing is complex and full of bullshit. It is not a community driven project. Once you have installed the self hosted solution, you get a user interface that asks you to upgrade to the enterprise edition in every corner and menu.

A self hosted version is better than nothing though.

Mattermost is fully licensed under AGPLv3 terms, and portions can be used under Apache 2 terms as well.

I'm not sure why people would say they're not open source.

It's true there's no community-led edition, but that's because no one has taken the initiative to create one yet.