The company said in a blog post:

Despite the incredible breadth of Extras developed for Skype, simply not enough people were using them to justify our continued support of the Extras programme. Unfortunately, effective today, we will shut down the Skype Extras programme.

The result of this decision?

We will no longer certify new Extras. However, all existing Extras will stay certified until their expiration dates and all unused test tickets will be reimbursed.

We will continue to distribute applications through the existing Extras Manager in Skype for Windows, but will no longer add new Extras.
All public API documents will continue to be maintained.
Skype will continue to support accessories via the Public API.
The Skype shop will continue to feature currently listed Extras.

It is a shame that the company still does not provide extensive access to their services through APIs. And they are now removing one of the few services provided to third party developers.