Google killed the YouTube video annotations editor last year, and in an "update" to the announcement now says it will be deleting existing annotations in 2019.

Update: We will stop showing existing annotations to viewers starting January 15, 2019. All existing annotations will be removed. … …As adoption of end screens and cards has grown, the use of annotations has decreased by over 70%. For this reason, we discontinued annotations editor in May 2017. This means you can no longer add new or edit existing annotations, only delete them.

Annotations were replaced by "cards", which among other things are integrated into the contemporary advertising and tracking infrastructure built around YouTube, and can coexist better with the underlying video on mobile platforms (especially iOS, which is under another company's control).

Though few loved annotations and they were often grossly hostile to viewers, it's also true that they were put to all sorts of legitimate and necessary purposes. None of which interest Google, so they disappear at its convenience.