Resize the drawing area of Unity and make margins above and below. You can set an image or a single color in the margin.
On the variables in
UnityIPhoneXSupport.cs, you can make the following settings.
Sets the size of the margin.
Set the color of the margin.
Set the image to display in the margin. If you do not want to display the image, leave it empty. Images must be under "StreamingAssets" Directory.