Some Cycles nodes changed as well. Just have a look at my thread about the glossy bsdf node. I don't know if the diffuse issue is by design or a bug. You could report a bug to devs but if it's not a bug then that's how it works now. The way you were using it was a hack anyway. Not that hacks are bad. I use a blank cycles node with nothing plugged in for eye pupils so that you will never see any geometry in the pupil ever. That is a hack that I use.

W10, Ryzen 5 1600x, 16Gb,RTX2060Super+GTX980, PP11, 11.3.740