The Google AdSense blog has announced retiring of Onsite Advertiser Sign-up, as it has not been performing well.
Google, AdSense, Advertisers, Features
We've elected to gracefully retire this this feature and focus our efforts on developing and supporting features that drive better monetization results for you. Call it time management, call it ROI, call it our unwavering commitment to our publishers. We want you to earn more revenue, and sometimes that means "sunsetting" certain features we created.
Please be aware that any links you've created that lead to a customizable Onsite Advertiser Sign-up page will soon re-direct to the main AdWords sign-up page. Of course, AdWords advertisers will still be able to target your individual sites by using placement targeting. Thanks for your support of this feature in the past. [AdSense Blog post].