ipad pro 11 inch 2022 back camera

The Portrait Mode to which you refer is an effect applied to pictures taken with the iPad's front "selfie camera":

Take a selfie in Portrait mode with your iPad camera - Apple Support

While this mode is not available for the iPad's rear camera, Apple does invite submission of comments and feature requests via its Product Feedback portal. For iPad and iPadOS:

Feedback - iPad - Apple

If you would like to see additional functionality released in a future version of iPadOS, this would be the most appropriate channel through which to make your request known to Apple.

This is odd, my iPad Pro M1 has portrait mode in both Selfie mode/camera and back camera mode. My iPad Pro was released in 2021 and your 2022 should have the ability to control Depth of field (blurry background) too.

What exactly are you trying to do? Do you have an example you can post of what you’re camera is currently producing?