From AdSense team:
We want to share an important update with you about the Onsite Advertiser Sign-up feature. You may recognize this feature as a link on certain ad units that reads, "Advertise on this site", or you may have included direct links to the Onsite Advertiser Sign-up landing page on your site. Unfortunately, this feature has not been performing as well as we had hoped, and in the coming weeks the feature will be retired.
As the AdSense product evolves, we occasionally re-evaluate aspects of the product to make sure they are performing well for our publishers and meeting our goals. We found that the Onsite Advertiser Sign-up feature was under-performing, and so we have made the decision to focus our efforts on features that we believe will drive the best results for publishers.
The Onsite Advertiser Sign-up link will no longer appear on any of our ad units. If you included direct links on your site to the customizable Onsite Advertiser Sign-up page, these links will soon redirect to the main AdWords sign-up page.
Thank you again for your support of the Onsite Advertiser Sign-up feature in the past, and we apologize for any inconvenience this news may cause.
Yours sincerely,
The Google AdSense Team
Google Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043