According to the docs you should be able to AB Test app icons now on iOS.
However, I can't figure out how to do it.
Here, it says you need to include an asset catalog that supports uploading alternate icons. https://help.apple.com/app-store-connect/#/devb53f12312
I've uploaded an app with alternate icons that the user can change in the app and that same binary includes an asset in the asset catalog for the alternate app icon. However, when I try to run an AB test on App Store Connect the icon field isn't present. I can only change the screenshots in a new treatment.