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.
i don't know if your surmises are correct. But my point is just that this seems irrelevant to Slack ending the deal. If they had stayed on the existing "grandfathered" plan, Slack could still be ending it. So I don't see the relevance. There's no reason to think they would be safer if they had stayed on a "grandfathered" plan that isn't even offered anymore, in fact it's often safer to switch to a currently offered plan if they're offering you a reasonable one, figuring they'll end the grandfathered one sooner than a currently offered one. That alone could be reason to switch in fact.
Right. The sensible thing to do would have been to develop a migration strategy as soon as you hear words like "grandfathered" or "first-year discount".
Not defending Slack / Salesforce. You just can't deal with them with that level of naivety.