Anyone else find it a bit ironic (?) that they say "an open platform is essential" but haven't open sourced their core code base? That feels like a bit of a misstep to me.
We have over a thousand people on our forums helping each other out, people are building installers for Mattermost in Puppet, Chef, Ansible, Docker, etc. (just web search to find them).
We're also getting help writing and updating our deployment docs (https://docs.mattermost.com/).
Slack is seriously user-centric, all the way down to how they run their Twitter.
It is the single most annoying thing about some projects - I get it, IRC doesn't show your emojis, but it just works.
I guess I'm also apart of the problem. :P
Come back to IRC, FOSS!
That being said I do believe that Slack is a poor choice of medium for a free project, mainly because of the limited history and no publicly searchable transcripts.
Can't seem to find the link, but you can see they will give discounts for non profits, up to 250 users. - https://get.slack.help/hc/en-us/articles/204368833
Its a poor medium, as it locks projects into a walled garden ecosystem.
(You might consider other aspects of openness more important, of course.)