What app is going to design to flow nicely with a iPhone Duo but not a iPhone 18? Unless Apple has a 1.4:1 ratio across their entire product line, app designers have to deal with variation.
Also, when you rotate the device, the ratio changes. This is especially common when unfolding a foldable.
People keep saying it’s so an app can resize to the full interior screen but none of the marketing photos actually have this. So its hard to say “what a genius masterstroke by Apple.”
Most have the closed screen plus another view. Or the lock screen which is a full photo open and a small section closed.
Im not saying its not possible but it doesn’t seem like that is their intention.
Folded and unfolded will both be designed for 2 aspect ratios. 1 for Portrait and 1 for Landscape. Opening the fold and rotating 90 deg will give you the larger version while keeping the aspect ratio