Even in XCode 12, it is still not possible to run a preview in landscape.
I list here two solutions I have found.. but note that with live preview, both options are ignored - we will really need to wait for an update from Apple to definitely solve this issue completely.

First solution

You can simulate it in this way:

struct ContentView_Previews: PreviewProvider {
	static var previews: some View {
		ContentView()
			.previewLayout(.fixed(width: 2532 / 3.0, height: 1170 / 3.0))
			.environment(\.horizontalSizeClass, .regular)
			.environment(\.verticalSizeClass, .compact)
	}
}

You can find the list with all iPhone resolutions at The Ultimate Guide To iPhone Resolutions and iOS Resolution.

Another alternative

When doing more researches on this topic, I have found this article: SwiftUI: Managing Previews.

It gives tips to use the preview in landscape, and also creates two previews, one for light mode and another one for dark mode:

#if DEBUG

struct Landscape<Content>: View where Content: View {
	let content: () -> Content
	let height = UIScreen.main.bounds.width //toggle width height
	let width = UIScreen.main.bounds.height
	var body: some View {
		content().previewLayout(PreviewLayout.fixed(width: width, height: height))
	}
}

struct LightAndDark<Content>: View where Content: View {
	let content: () -> Content
	var body: some View {
		ForEach(ColorScheme.allCases, id: \.self) { colorScheme in
			content().environment(\.colorScheme, colorScheme).previewDisplayName("\(colorScheme)")
		}
	}
}

#endif

Once added these helper functions, you can use them in this way:

struct ContentView_Previews: PreviewProvider {
	static var previews: some View {
		LightAndDark {
			Landscape {
				ContentView()
					.environment(\.horizontalSizeClass, .regular)
					.environment(\.verticalSizeClass, .compact)
			}
		}
	}
}