It's not safe for anyone with walnut allergies. I worked in an animal shelter where they used this walnut litter in the main cat rooms. I am not allergic to cats but would wheeze after a few minutes in the cat rooms, before I ever knew it was walnut litter (I thought maybe I was allergic to cats after all, but wasn't). Plus hives, so many hives, swollen eyes, and a runny nose. They switched litter to pine pellets shortly after I brought up the allergy issue, as it could become a public health issue. After the switch, I had no more reactions in the cat rooms.
With so many other (and better) options out there, there is no reason to use walnut litter if you have any sort of nut or mould allergy. It's great if it works for you, but I would never recommend it to someone with nut allergies.