Hack Club was on a grandfathered free nonprofit plan, but switched to a 5k/year on earlier this year under a special deal with Slack. Now the price is increased to 50k one time, and then 200k a year
There is no obligation to provide the "grandfathered" free plan forever, they could have taken that away too. Presumably they switched to get some features they wanted that weren't on their old plan. Slack can take away the old deal as easy as the new, staying on the old is no guarantee Slack will let you keep it forever.
Right - there is so much context missing. Extrapolating just a little, it looks like the story is something more like "We had too many users for our existing free plan, so we switched to a paid plan that had a first-year discount of 97%. We didn't really pay attention to the fine print, and now we've got a nasty letter demanding lots of money."
Not defending Slack / Salesforce. You just can't deal with them with that level of naivety.