We're using active directory integration, so when a user leaves they are deleted from AD (and before anyone suggests it, AD is managed by another group so I can't change their policies), this also deletes the user from Confluence.
Any content they have written is still there, but it is longer possible to find that content based on author it seems. The page will show Unknown User (original_user_id) as author. Even just being able to search by Unknown User would be helpful.