The process to reproduce:
Create a new extension with Xcode 16 beta.
Run it on the simulator or any iOS device with iOS 18 beta installed.
Open Safari on your Mac, then open Develop -> Device you use.
Unable to access these menus for iOS 18:
1. Extension - Extension Background Content
2. Extension - Extension Pop-up Page - popup.html
However, for iOS 17.4, I can see these options in Develop -> Device iOS(17.4).
I have another extension on the App Store that is not working on the iOS 18 beta, but I am unable to debug it because of this issue.