Thinking maybe an unwanted app upgrade caused it, I removed the YouTube app from my Firestick and sideloaded the following versions:
* AmazonYouTube 184.108.40.206
* AmazonYouTube 220.127.116.11
* AmazonYouTube 18.104.22.168
But they all exhibit the same behavior. I verified the versions were all as expected every time
I checked all the YouTube settings everywhere but can't find where I can tell it to zoom 4:3 videos. Anyone? Why in the world is this not a simple setting like every video display app has had for like 15 years now?