I've found message retention to be a "checkmark feature:" It's technically there, but it's not really achieving the goals you'd hope for message retention to help with. You'd want to be able to say "Don't retain messages more than 90 days old except for the rooms I, the admin, specify." But:

* There's no API to set retention settings or defaults

* There's no API to audit retention settings for private channels

* Non-admin users can change retention settings for DMs and private rooms

* There's no way to enforce that private rooms and DMs get a maximum retention setting