The plastic curtain has added plasticizer to make it soft and flexible. Plasticizers are usually small organic molecules. Over the course of many years in the summer sun, the plasticizer dissipates. You don't see the bottom layer which is a glass-fiber reinforced thermoplastic. On top of that is a layer of polyurethane foam.
ABS is an acronym for a block copolymer made of acrylonitrile, butadiene, and styrene.
PVC blended with ABS effectively does the same thing as PVC with added plasticizer, but in this case, the ABS doesn't evaporate so the dashboard lasts a lot longer. There was once a debate over the effectiveness of the product Armor-All . One assertion was that, instead of replacing softening oils, Armor-All actually brought oils to the surface causing them to evaporate more quickly. Of course, another study showed the complete opposite. Vaseline is favored among many automobile enthusiasts.