I read the message retention policy as a true delete for anyone not wanting to get their previously private messages grandfathered into a newly non-private policy.

Slack warns you for a channel that setting the retention policy is "truly, permanently deleted. These messages can't be restored or recovered, even by Slack"

https://i.imgur.com/63x2oee.png

https://get.slack.help/hc/en-us/articles/203457187-Customize...