Solidify Modifier, Subsurface Scattering, and the Principled Shader I've read/heard elsewhere that the Subsurface Scattering in the Principled BSDF Shader only works if a Solidify Modifier is present. Is this correct?

If I create a flat Mesh Plane e.g. it would make sense that I need to apply a Solidify Modifier for Subsurface Scattering via the Principled BSDF to have an effect. 

However, If I created a Mesh Sphere -- or Cube or any non 2D or non-flat object -- do I also need to add the Solidify Modifier for the Subsurface Scattering via the Principled Shader to have any effect?
No, i dont think you will need it for a solid object. if you apply a solidify modifier, you will see that all it does is just add another layer of faces to create a closed mesh, which you already have with a sphere.

You can also just try applying a subsurface scattering material to a sphere and see what happens.