A previous employer who shall not be named had a policy that all Slack DMs older than 2 weeks were immediately deleted. Can't find any smoking guns if the gun has been incinerated.
It also had the nice benefit of forcing people to store actual knowledge in Notion where it could be organized and more easily discoverable.
Anyway, all this to say - there are many reasons to purge slack history that aren’t nefarious (even if the practice is ill advised by some)