I understand the value of the historical information to the handful of people that wish to access it. However, with every passing week, it becomes more and more out of date. Many times the information presented was already obsolete compared to "then-current" releases, and could and has lead users to all kinds of problems, including bricking devices. The information on devices that weren't officially supported a year or two ago is often "completely wrong" on the obsolete wiki, especially when flashing techniques and/or MTD partitioning has changed.
Users should be able to search on a major search engine and get a "correct" answer to their question. They certainly shouldn't have to jump through hoops to then go to the "current" wiki and try to guess the name of the equivalent page.
Here's a good one
https://www.google.com/search?q=openwrt+telnet -- first result
https://www.google.com/search?q=openwrt+login is just as bad and misleading
How many times a day does tmomas or one of the frequents here on the forums have to redirect users who reasonably searched using an Internet search engine for a resolution to their problem to the "right" page?
Short of providing a link at the top of every page on the obsolete wiki to the "new" page (which is a huge effort, as there isn't a straightforward mapping of old to new). I don't think that more "warnings" aren't going to help. Warnings haven't significantly dissuaded people from referencing those pages here time after time. Who knows how many more refer to them, not even realizing that they're looking at outdated information?
As long as those pages are still indexed by the major search engines, they will likely remain at the top of the listings for years to come.
For the handful of people that want access to those pages for historical purposes, the links can still remain valid and they can use the wiki's search or old link references.