Yes, Coach used plastic (actually nylon) zippers in many bags starting in the late 1980s. They still do.
Check the "Authenticity Guide Facts & Myths" guide in the set of six Salearea Coach Guides.
http://www.ebay.com/gds/Coach-Authenticity-Guides-Facts-and-Myths-/10000000062607882/g.html
"THE NO PLASTIC ZIPPERS RULE
Nonsense! Plastic zippers were used in many bags from at least as early as the 80s and 90s. Most were used on the inside pockets especially in the Lightweights line, but the early 1990s Sheridans and probably Dakotas used them on the outside too. There were plenty of cases where Coach wanted to either reduce the weight of the hardware or color-match the zipper to the leather or lining colors. And modern Coaches still use plenty of plastic zippers..."
Thank you Hyacinth. I thought I'd printed all the recommended guides, but I don't remember this one. I appreciate your help!