At the end of the Mank (2020) movie, the Windowbox effect used on Oscar award show:
Why the Windowbox effect used on Oscar award show?
Everything has been compressed and made to sound like the 1940s.
Like the cue marks, it's intended to make the image look old. As this time, many movies were part of an entire show, with cartoons, news and more. I remember news being black and white in 4:3 format, so we could also see that stamp border around the screen.
Here are some pictures from the movie, with the same characteristics: