It's been like this for several years. It's possible your account is old enough that your memory is correct.
I did it to discourage spammers. I wanted to make sure they got zero clicks from their posts.
Would it make sense for high-karma users to be able to endorse dead links?
Yeah, that might be a good idea.
about 10% of the dead links on the new page are actually rather intersting, but I end up having to search for them on Google. Sometimes the filters miss good stuff, eg I found a good long-form article on Buzzfeed last week but it was killed automatically upon submission. Well, can't win them all.