Short answer
A single lick of pistachio ice cream is low risk, but it's not something to offer. The main concerns are the high fat and sugar in the dessert, which can cause stomach upset, and the salt on the nuts. Pistachio shells are sharp and a choking hazard, but a lick likely won't include shells. Watch your cat for vomiting, diarrhoea, or signs of abdominal discomfort over the next day. If those appear or your cat acts unusually tired, contact your veterinarian for advice.
Pistachios — Food safety facts
Feed with care · Cats · full breakdown on the linked food page
Usually salted, and the shells crack into sharp pieces. Avoid.
The full verdict, portion guidance and serving notes are collected on the Pistachios food facts page.