Piecing together various clues across the internet from techniques that didn't work or were ungainly, I've figured out a tiny bit of code that might help others who, like me, have been trying to figure out a way to determine an iPad's orientation for presenting different UI arrangements based on whether a user is using it in landscape or portrait. (SizeClass works with iPhone, but iPad thinks both P and L orientations = .regular).

For example, if you have a ContentViewPortrait struct for portrait orientation and a ContentViewLandscape struct for landscape orientation, then you could use a ContentView struct like this:

struct ContentView: View {
  @Environment(\.horizontalSizeClass) var orientation
    var body: some View {
        GeometryReader { ruler in
            if ruler.size.width < ruler.size.height || orientation == .compact {
                ContentViewPortrait()
            } else {
                ContentViewLandscape()
            }
        }
    }
}

@Environment(\.horizontalSizeClass) var orientation and || orientation == .compact are (only) necessary if you have the "Support multiple windows" option checked in the General/Info panel, because Slide Over and Split View seem to come along for the ride.