For iOS 16+

There’s a new native modifier for creating inner shadows, which can even be used on strokes like this:

    var body: some View {
        Circle()
            .stroke(.white.shadow(.inner(color: .gray, radius: 10, x: 2, y: 2)),
                    lineWidth: 50)
            .frame(width: 300, height: 300)
    }

The result might look like this: