Slack just announced they're deleting most of the message history of workspaces on the free plan.
They are not giving groups a chance to save their older messages. We still only have access to last 90 days.
Options are: buy a paid plan for a month and use it to download your archives, or abandon your workspace history.
May be able to hack this with free trials but either way it's a reminder that VC funding will run out and the free things you've come to rely upon will be used to exploit you.