The early AI-S lenses are mostly (all?) metal, I think the beauty ring of the AI-S 50/1.4 and AI-S 50/1.8 (long nose) are plastic but that's about as far as it goes. The series-E lenses from the same period did use plastic. By the mid 1980s the series-E line was discontinued, or rather, merged with the Ai-S line, and you start to see series-E features appearing in Ai-S lenses. The most obvious is the newer pancake 50/1.8 which has a mostly plastic exterior, and is really a remodeled series-E lens. Zooms from that period also have plastic parts, the Ai-S 35-70/3.3-4.5 has a mostly plastic exterior and the aperture ring of the 28-85/3.5-4.5 is also plastic.
AI-S lenses introduced in that period also tend to have a black anodized finish on the metal parts, like Series-E lenses, rather than the black enamel finish of earlier AI-S lenses. For example, the AI-S 100-300/5.6 at first glace seems like a longer version of the 80-200/4, but the design and finish is really closer to the Series-E 70-210/4.
Overall I would say there was a small decline in build quality which already started during AI production. Very early AI lenses were engraved LENS MADE IN JAPAN which was filled in white paint. This soon changed to unpainted MADE IN JAPAN. This is not really a reduction in build quality but it's less "luxurious" and shows Nikon was trying to reduce costs and simplify production. Early AI lenses also have 5 screws in the mount, but this dropped to 3 screws for most primes 85mm and shorter. Having 5 screw probably wasn't really needped for small lenses so this was a sensible simplification in production. With the transition to Ai-S most lenses up to 135mm have only 3 screws in the mount.
I can only guess there were similar internal changes to streamline the assembly process and reduce costs. I don't know much about the internal workings, but I know that early production of the Japanese AI-S 50/1.8 pancake lens used brass focus helix, but this later changed to aluminium, so it's possible the same occurred with some other AI-S models as well.