I jumped up and down about this to Slack over the course of a few emails, having some paid workspaces and a couple on the free plan that I use(d) for family. Eventually I got one of those "Hi this is such and such, stepping in for the other person" emails, with a link to this app that can export everything (including private messages):
The export is in a proprietary JSON format and only contains links to attachments, plus it has a limit on the number of messages you can export if you don't pay for it. But after jumping through various hoops one afternoon I was able to export all the messages and attachments from a family channel that we've had going for a few years - since our kids were first active on computers. The total number of messages was about 3500, but we would have lost the vast majority of them with Slack's new approach.
