Earlier quoted context omitted.
> pay or lose months or years of history. You can download all your workspace data now, includes chat history. Chat history on free plan was already limited, so I don’t think anyone who was aware of that was using it as an archive.
You can’t download private messages, however.
https://export.fountstudio.com/
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.