Currently, if you disable/delete a user from the RC admin console, all conversations with this user disappear. This includes direct 1-on-1 communications, as well as ad hoc groups (not just their messages, but the whole group disappears). Often times, these groups contain pinned/bookmarked messages and attachments that are important.
It would make more sense if the past employee's content was left intact, but their name/avatar/profile simply denotes their off-boarded status. A group shouldn't disappear until the last person in the group no longer has an account.