Annotations were a common tool for content creators in the early years of YouTube. If you would not be familiar with the term, annotations are the colored boxes that appear on the screen during a video with additional information or links to certain websites, other videos, etc.
If you used the annotations correctly, they could certainly add value to your video. Often, the annotations were used too much, so that some videos became a colorful rainbow. You also had to click away some annotations to get them out of your image, and while watching a video, people would rather not engage in the constant removal of annotations. The editor for these annotations has already been discontinued in May last year.
YouTubers could from then on only use the new “Maps” and “Lock screens” to show more information and links in the picture. Since the introduction of these two functions, the use of annotations has also declined visibly. So it has not been possible to create new annotations for a while, but the existing annotations still appear in the meantime, but from January 15, 2019, YouTube will say goodbye to the annotations forever.