Slack has raised our charges by $195k per year

Since you're a nonprofit that teaches coding, it could be a great time to consider self-hosting a FOSS chat tool.

Suggestions: Campfire [0] or Zulip [1].

Also, if the data in chat is being held hostage, the org might be using chat wrong. Right tool for right purpose. If starting over, perhaps consider if it would make sense to put that documentation or whatever it is that will get "lost" from Slack into a wiki or repo or other appropriate tool?

Big empathy, though. It must be pretty crushing. But that is why serious geeks have long been for FOSS.

  [0] https://once.com/campfire (recently became FOSS) 
  [1] https://zulip.com

I'm in Hack Club, the team is moving all of us to self-hosted Mattermost. It is unfortunate that we have to re-code so many things though.

I've never used Mattermost before today. After checking out their site, I can see they are also a for-profit company. What does Mattermost offer that Slack does not, other than a bill lower than $195K/year?

You can deploy it self-hosted without paying any fee, so you control your data much more.

Last time I checked they cripple the self-hosted version, asking to subscribe for enterprise plan here and there. Source: deployed their chat locally a couple of weekends ago. Overall, I liked their Slack clone, they this one was a red flag to me. Now I’m not sure we want to deploy this, but I know very little alternatives. Zulip, but it cripples its self-hosted version too. It allows just 10 mobile users (notifications). Maybe Matrix it is then, but it’s not very suitable for airgapped company-wide deployment.

> Maybe Matrix it is then, but it’s not very suitable for airgapped company-wide deployment

Element is literally built for airgapped company-wide deployments - this is precisely what https://element.io/server-suite is? It was originally built to install onto SIPRnet; it's been airgap-first since day 1.