Google announced recently – or rather quietly updated a YouTube help page from years ago – that all existing annotations that were created before the annotations editor was discontinued will be removed from videos by early next year.
Back in May of 2017, Google discontinued the annotations editor due to creators putting an unreasonable amount of annotations on their video uploads. These colored blocks caused viewing to become obstructed and, quite frankly, laborious – videos looked disorganised, and the pop-ups were distracting.