Looks like this is an opt-in, supplemental crop mode, for streamers who have stuff that would work well for vertical orientations. Talking head stuff, live solo-artist music, etc.
The existing content experience is still there, but if you’re standing on the subway, and you’re watching a streamer react to a press release about a game, you can turn your phone and watch them babble in landscape while you hold the poll.
Well, there’s your problem