You And Me Academy has a look into creating a shadow catcher in Cinema 4D Redshift.

One of the most critical passes might be a shadow pass. You can render shadows on a separate image to later composite it back into the scene for more control. The shadow catcher also lets you have objects appear as if they are in the stage without having to create the scene.

How to Create a Shadow Catcher in Redshift.

Check out this latest tutorial from Joe Lawrence (The You and Me Academy) that shows how you can quickly create a way to render only the shadows on to a plane. Previously with Redshift (as with many other renderers), this was facilitated through a material. Now in Cinema 4D, it’s as simple as adding a Redshift Tag.