https://wccftech.com/iphone-only-apps-now-work-in-landscape-mode-in-ipados-15/

Great news everyone, you will be able to use the Instagram iPhone app, or any iPhone app for that matter, on iPadOS 15 in landscape mode.

There are a ton of new features packed into iOS 15 and iPadOS 15, and right now we stumbled upon something which is going to make a lot of users happy. This feature allows you to use iPhone apps in landscape mode. Right now, if you have iPadOS 14 or earlier installed, you have to use apps in portrait mode, with the iPad itself held up straight.