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!

Cool! Where should I send bet money?

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.

Have your Slack chats been subpoenaed yet?

If not, you are very lucky that you are being sued by people with incompetent attorneys.