That documents retirement of an API. Someone visiting "search.twitter.com" in a browser wants to see a web page. It would seem sensible to redirect them to a place where they can do that.
Not that I went there often, but I always got to search via "http://search.twitter.com"... earlier today it was returning a 404 [1], now this weird API deprecation message. I just wanted to type a query in via my browser!
Embarrassing.