I know there are several existing threads on this topic but things keep changing.
The release notes for Xcode 26 beta 3 have the following statements for a couple of resolved issues:
Asset Catalog
Fixed: Unable to set Icon Composer icon as alternate iOS icon (153305178) (FB18025356)
Icon Composer
Fixed: Icon Composer icons back deploy to older versions of iOS, macOS, and watchOS with inconsistent rendering. (152258860)
I had a working solution under beta 1 and beta 2 for both of these. But under beta 3, I am now seeing the new glass icons for my app when running on a simulated iOS 18 device. This is happening for both the main app icon and any alternates. This contradicts the statement that beta 3 fixes this issue.
There is no documentation (that I can find) describing how you are supposed to support old icons for iOS 18 and new glass icons for iOS 26. There is no documentation for how to support alternate glass icons for iOS 26.
What I'm doing at the moment (that worked before beta 3) was to have the normal iOS 18 app icons in the Asset catalog and to have the new glass icons added to the project. The filenames for the glass .icon files have the same name as the app icons in the Assets catalog. This worked under beta 1 and beta 2. And despite the Xcode 26 beta 3 release notes stating that Icon Composer icons no longer back deploy to iOS 18, I'm seeing the opposite. Beta 3 now does the opposite of that statement.
Does anyone have a working solution that supports old iOS 18 app icons and new iOS 26 glass icons using Xcode 26 beta 3?
Note, all of my testing is with simulated iOS devices and I'm running Xcode 26 beta 3 under macOS 15.5. Maybe that's an issue?