In case it's helpful to anyone else, I managed to get past the problem I was having with the new style icon. It turns out, all I had to do was rename the AppIcon.icon file to anything else (within the Xcode 26 GUI) then rename it back. Apparently, I added the file to the project in Xcode 16, so it wasn't flagged properly as an icon asset. Later, I had a separate problem with the same symptom. This time I added the .icon file with Xcode 26, but in the dialog, I neglected to check the box for the target, so it wasn't added to the "Copy Bundle Resources" build phase.

Well, I don't know what I'm doing wrong. I'm using Xcode 26 Beta 6 on a macOS 26 Beta 6 VM, with macOS 15.6.1 as the host. I can build the TahoeIconsTest project, pointed to by @superpixel, and it works just as expected, the LG icon is used on macOS 26 and the old icon is used on 15.6.1. I do note that the .icns and .icon files are not in the app bundle , but there is a .car file. The Info.plist only has CFBundleIconName set (to AppIcon). However, I'm just not able to replicate this outcome with my own app. I added the "secret" flag and all the other asset compiler options are the same as the test project. But my build has all three files, .icns, .icon, and .car in the bundle, and my Info.plist file has both CFBundleIcon* parameters set to AppIcon. The result is both OSs use the old icon. I just can't see what's different.