More details here:
https://devtalk.blender.org/t/lets-finally-fix-the-shadowcatcher/691
If any developer from the #render_cycles team is interested on this, please bring the content from devtalk to here, in a tangible way. If not we can just remove this task, honestly.
Small note: "An efficient GPU implementation is not straightforward, though it can be solved probably" -- Brecht