Support both orientations, if possible. I know not even iOS itself does it, but it hasn't always been like that. You'll create a more robust UI that will be easier to port to iPadOS. And especially, don't force your users to rotate their devices.

The Youtube app is playing a video. When going fullscreen it forces you to change the orientation of your device. This is not recommended. It could instead go fullscreen in portrait and then give the user a choice to rotate the device in landscape mode if they wish to do so.