We are running a small experiment where 0.3% of viewers will see an auto-generated thumbnail, instead of your custom thumbnail. We are not removing the ability to create your custom thumbnail, but we hope to gain insights on auto-generated thumbnails for the future.
Here's an idea; create YouTube Classic where you leave people alone and also create YouTube New where you control thumbnails, sub feeds, notifications, etc. And see which one does better, eh? I think I know which one is going to do better.