Yep, in the 80's we always called it "nitrous", and NOS was a brand name (and still is, see NOS).

I never heard anyone call it generically "NOS" except for people reading the NOS stickers on the side of a car ... although I do recognize that nowadays people and companies are using it generically. However, the commercial enterprises might possibly face legal action for misusing Holley's trademark.

It's very much like the misuse of the name "Coke". Yeah, you might call any brand of cola a "coke", and people will usually know what you mean ... but to other people they want a Pepsi and definitely not a Coke, so they are careful to state exactly what they want.