In the real world, those tend to break somewhere around the second or third CMS upgrade.
"I have trouble believing any CMS author could fail to support them"
It almost sounds like you are assuming that The Onion has some sort of duty in maintaining ancient URIs. IMO, they should maintain their redirects as long as deem it necessary - there is a cost in doing so after all. Sure, the cost of not maintaining outdated URIs may or may not outweigh the cost of losing vistors, but that is for them to decide, not the peanut gallery.
I'm sure I'm not the only one that isn't so entitled that I wouldn't forgive The Onion if the links from my 2001 bookmark.htm export fail to land on the original content.