I wanted to create transparent images I could use as layers in Photoshop. The text and/or objects needed to cast shadows without visible planes.
There is no need to add a material to your shadow catcher plane! The plane will be rendered transparent, but with the shadows cast by other objects.
Imported into Photoshop with a solid red background. In Blender, I used a vertical plane, and a horizontal plane to show how the plane orientation can affect the results, cause by the shadows and camera perspective, when the object is made of a glass material: