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.
I create custom thumbnails so that I don't have to do deal with YouTube picking a random shot from my video and now, thanks to this, some of my viewers are going to see a random shot of my face as the thumbnail... just wonderful