Yeah, I didn't understand and jumped straight to the comments to see if there was an easy explanation here. Guess I should have clicked "How?" at the top.
Even after reading that page, I didn't understand that this was a suggestion to start regularly archiving old versions of the site and only sending visitors there if their link didn't point to anything on the current site. Instead, I thought the idea was that, for software reasons I don't understand, web developers commonly changed the subdomain name for the main site and this was just a method for reducing the number of broken links when such a change was made.
4042302.org definitely needs better explanations.