According to the Slack HQ account on Reddit, the situation has been resolved: https://old.reddit.com/r/Slack/comments/1njuchb/why_is_slack...
>We made a mistake. >This was the result of an oversight in our billing process, and we are returning Hack Club to its previous nonprofit pricing while we work with them directly to ensure their workspace remains fully accessible. We value the work Hack Club does to inspire and educate young people in coding and technology, and we regret the concern this situation has caused. We will be reviewing our billing and communications processes to provide nonprofits clearer guidance and adequate grace periods as they grow.