I see what you mean about the warning. I think the vBulletin software brings up the warning automatically. We would need to attach a post or a banner to that thread to do what you propose and that would take manual intervention the way the board software works presently. Since there is good info in some of these old threads, the flash warning only comes up when you try to post. They are still good for reading and that would be a pain in the you-know-what to get that dialog every time you opened the thread just to read it.
We don't have a dead thread category. We just look at the dates.
We don't see these things as problems needing solutions.
"The irony of the Information Age is that it has given new respectability to uninformed opinion." - John Lawton, speaking to the American Association of Broadcast Journalists in 1995