Back in March 2017, YouTube announced that it would be retiring annotations, those boxes that pop up during a video with links and additional information. It discontinued the annotations editor in May of last year and soon all existing annotations will be going away as well. The company added an update to the help page announcing the end of its annotations editor, saying, "We will stop showing existing annotations to viewers starting January 15, 2019. All existing annotations will be removed."
Annotations didn't work with mobile and in response to creators' call for annotations that would, YouTube created Cards instead. It also introduced End Screens in 2016, and as those two features became more popular, YouTube said annotation use decreased by more than 70 percent. The company says that compared to annotations, Cards and End Screens are more engaging and easier to create, along with being functional on mobile.