The television adoption of widescreen was sort of the reverse trend of how it was handled in the 1950s, and certainly has a good number of obvious variables.
At the start of widescreen television, the typical method on ABC was to center cut (crop) for its standard definition feed. And widescreen televisions at the start of ABC HD broadcasting were of course, only a select few. Therefore the creators of ABC content would typically compose everything for 4x3 and "the 16x9 sidebars", as they were called at the time, were simply "filler". I worked in Operations at a Hub facility during this timeframe where we monitored 4x3 SD air content along with 16x9 HD content simultaneously on monitors right next to each other, which made for great comparisons. The wilted set piece plant often made its appearance in the extra 16x9 area, or a portion of a head, nose, or what have you .. as those shooting were focused on their primary 4x3 viewing audience. There were also some surprises, like a few Peter Jennings specials where two separate masters were created. A good example of this was when (roughly) 4x3 archival photos were used, the 4x3 SD version would show more picture info, while the HD version was cropped for 16x9, but not a simple center crop. Only 16x9 HD footage used in the specials were actually "cropped" in the SD version. The same went for text and graphics. A few of the popular shows (LOST as one example) also had special 4x3 and 16x9 edited versions, at least when it came to the opening and closing on-screen text in the first season. As widescreen sets became more popular, so did the 16x9 framing. Now the landscape is totally reversed, where there is no longer an SD ABC feed. And 4x3 compositions are, for the most part.. very secondary and sometimes totally discarded.
Having also monitored NBC content during the transition to widescreen, I was also impressed how NBC handled their SD content. With the exception of some dramas and news content, NBC would typically letterbox their SD downconverted content. This hypothetically allowed content creators to compose in widescreen without fear of their work being hacked for 4x3, or at a minimum, protected for the primary runs when shown on NBC. The network was also was the first to push for AFD (Active Format Description) in their HD feed metadata, which for the more advanced SD set top boxes, could tailor the downconverted image per the wishes of the content creators. For example, news or sports could be set to center cut in 4x3, so the 4x3 displayed a larger "full" image on the 4x3 screen, while other titles were coded to letterbox to preserve the widescreen compositions. AFD as used by NBC did not affect widescreen viewers in any way. Actual implementation of AFD in home gear had a slow start, but it showed the network cared about how everyone viewed their content, no matter what the AR was. NBC also eventually ditched their SD only signal, and with widescreen displays now being cheap and prevalent, the need of AFD is nearing its end.
Again, I tend to consider widescreen television a different discussion of sorts, compared to great research and interesting ongoing discussions Bob, Jack, and the rest of the HTF have been discussing here in this thread, but thought this was worth a quick mention at this time.