Annotations were supposed to be a helpful tool in helping the audience watching a YouTube video get more information or follow a link but the feature did not work for the mobile users, and, as the vast majority of YouTube usage nowadays happens on mobile, it prompted YouTube to create the Cards and End Screen features instead.
Soon after their inception, those two features became more popular than the Annotations, whose use decreased by over 70%.
YouTube announced way back in March 2017 that it will, eventually, retire the Annotations feature but the decision has been made final just recently, via a blog announcement.
“As adoption of End Screens and Cards has grown, the use of annotations has decreased by over 70 percent. For this reason, the time has come to discontinue Annotations Editor. […] Existing annotations will continue to show when using a desktop computer. We wanted to give you advanced notice so you can adjust.”
Existing Annotations will completely stop appearing to users starting January 15th.