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.
Another thing that this new idea you guys are trying out on a trial basis will do is make your various feeds an unidentafiable mess to look at. Thumbnails will look all samey. It would be harder to quickly id at a glance videos you are looking for.