How do you do the infinite backdrop trick in Blender 4+? (cycles)

Hello.

As the title - to get a reflectionless backdrop I used to use a surface with roughness 1 and specular 0.

How do you achieve this in Blender 4.0? Specular no longer seems to have a value, but rather an IOR. I’ve tried setting various things to 0 or 1, but can’t get the old effect.

Can it be done with the principled shader or do I need to use another shader?