There is a hack to use the Redshift State node to fake a Lambert shader but is does not take shadows into account nor lighting only a vector from a null object. When I get back I’ll see if I can post a link to the video.
As the Devs explained the problem is that Redshift shades once, unlike Arnold, so it can’t do this trick. OSL may help with creating toon shaders but it seems it’s a looong way away. This was all promised a couple of years ago now and they’re in the process of rewriting the whole Redshift core again.
I bought Redshift for GPU based Toon Shading but they haven’t delivered in 2 years and now they’ve stopped mentioning it.