I think it's more complicated than what has been stated thus far.
We are Dish Network subscribers and have a 16:9 HDTV in our breakfast room. The picture ratio can change from channel to channel, program to program, and commercial to commercial within the same program. Sometimes it's the exact "letterbox" aspect ratio as the screen, but shrunk way tiny in the middle of the screen with giant black space around ALL of the edges. Like, if I wanted a 10" screen that's what I would have bought. I don't get that at all.
Local news broadcasts may fill the screen or leave sidebars. This isn't because of how the picture is being formatted at the source, because the sidebars are actually cutting off part of the picture. A good example is the weather forecast. When they provide the 7-day forecast with graphics from left to right, the first and last days of the forecast are cut off by the sidebars. No changing of settings on the TV itself helps. The most reliable sidebars come from the local PBS station. Just about everything they present is cut off.
OTA is a little better, but it depends on whether the image is "Set By Program" per the digital converter box on-screen display. On my 1950 Admiral with 10" screen, the best option is to set the converter to zoom when it allows for it. You have to give up the periphery though, otherwise you'd not be able to see much detail.
We have a 40" HDTV in our bedroom, and that one is hooked up to an HD descrambler box from Dish. Even with that we see variations, and it's not about old vs. new programming.
I've given up on any remedial settings for the sets that are hooked up to Dish.
It seems to me that there's no standardization of broadcast methods since 16:9 became the standard ratio for new TV sets, or since OTA went digital, or among satellite channels. It's all quite whimsical. I would be really pissed if I were a heterosexual cave man who paid big bucks for a 100" screen to watch football, only to receive a picture framed by fat sidebars.
I'd say it's time to revive the test pattern, but they would probably be delivered in as many different ratios as there are stations.