I'm willing to bet money that for every company that deletes emails older than some age also deletes Slack messages, and deletes then sooner.
I speak as someone who worked for a company where email got archived and then deleted after 6 months, and currently works at a company where everyone only uses Slack, and Slack is purged after 30 days.
Isn't that just a company policy? At some-point, the lawyers will get clued-in and require that even on Slack and other company-wide chat there be a limited retention window!
Do the people who wrote the email policy know that Slack is permanent? That sounds like the kind of thing that could easily be not-understood by legal.
I speak as someone who worked for a company where email got archived and then deleted after 6 months, and currently works at a company where everyone only uses Slack, and Slack is purged after 30 days.