How to exclude code from AppExtension? @available(iOSApplicationExtension, unavailable) does not work

Question

I use a custom tool class across different projects which provides some basic, shared methods.

While I can use this class in most projects without any problem, I cannot compile one project any more, which includes an AppExtension / Widget.

This code fails:

class var applicationInterfaceStyle: UIUserInterfaceStyle {
    // deactivated method code and only return default value...        
    return .unspecified
}

'UIUserInterfaceStyle' is only available in application extensions for iOS 12.0 or newer

I tried to add @available(iOSApplicationExtension, unavailable) to this message, but the error is still the same:

@available(iOSApplicationExtension, unavailable)
class var applicationInterfaceStyle: UIUserInterfaceStyle {
    // deactivated method code and only return default value...        
    return .unspecified
}

Why doesn't this exclude the method completely from being compiled for the extension?

Xcode itself proposes to fix the error by adding @available(iOSApplicationExtension 12.0, *). But this does not work as. Additionally I would prefer to exclude the code completely from the extension. How can this be done?