If you really want to get to submissions that have been marked dead without any upvotes a simple “site:” query usually helps. The domain is still shown, even with dead links.
No 'link juice' or anything would ever flow to those sites.
And I doubt the folks that look at the site using 'showdead' need protection from malware or spam sites.
The one reason I can think of is that any clicks from HN would lead to those users re-posting under different accounts because they got some traffic. And in that way it might be beneficial to really get rid of them.