This was the same thing as forums a long time ago. Unfortunately they are dead now. However, it wasn't uncommon to have a special member class for people who paid into some tier (usually less than a cup of coffee) to support the host.
It's easy to forget that while you're freeloading someone is paying the bill. Slack is just finally forcing free tier user's hands. For people like me who use a slack channel once in a blue moon outside work this change is effectively a no-op. The people who were hosting vibrant communities on the free tier now have the hard job of cajoling people who have no intention to pay into paying. I don't see a problem with this frankly. Slack is still footing the bill for storage/compute/etc and if the accounts show no real intention to buy into a better tier sometimes you need to twist the screws a little.