Yes, it's a week or so old and means that user flags killed a story or comment.

I don't understand your question about retroactiveness; can you explain?

Has there been a change to the number of downvotes needed to make a post "dead"? I've had showdead on forever, and it feels like I'm seeing many more posts that have been deaded by voting alone.

Essentially I'm asking if it applies to all posts or only posts after the feature was added.