We can do searches for content by user with the syntax
xxx equals the user number. However, this does not work with deleted users.
This could be useful for the same reasons the user-based search is useful in general, of course. In the case of deleted users, this can sometimes be especially useful, though; for 10k and diamond users doing moderation duties, in particular.
In fact, this would also be a great thing to simply link directly to on the moderator's page for deleted users (perhaps in conjunction with showing deleted posts)