The YouTube Annotations Editor is being discontinued.
As of May 2 you will no longer be able to add new YouTube annotations to your videos or edit existing annotations. All you’ll be allowed to do is delete existing annotations.
If you have any existing annotations they will continue to be shown to people who watch videos on a desktop device.
In a blog post, YouTube announced the Annotations Editor, which launched in 2008, is being shut down for two main reasons. Read the article for details.