They opted out of moving to Mattermost. After the uproar that saw the slack ceo and cpo going to hacker news to do damage control, slack gave them 5 years of free "enterprise +," their highest tier. Apprently the slack ceo and other exec also called and apologised to the hack club founders.
It is disappointing that hack club opted to stick with Slack, but i get why they may be overwhelmed and maybe feeling the effects of a charm offensive. I'm betting, although nothing has been announced, that Slack and its Salesforce billions, is going to make a donation down the line too.
Hopefully they spin up at least some parallel infrastructure. Some of the teens were commenting that this was an eye opening experience for them about owning your data, so at least that will likely stick.