Slack previously stored history beyond 90d but didn't allow it to be accessed until the workspace was upgraded to a paid subscription. (For example, Slack recently ran a promotion where workspaces could get a free one-month trial; for the duration of that trial, users could view the full history of the workspace.)
The key difference: They showed onlymlimited amounts, but kept the data. So once you paid you got the full archive. Now they delete the old data and you can't pay to get the old stuff back.
Considering that I have somengroups where we'll never pay this is a good change l.