Here is their official response to why they ended this program:
Publix originally rolled out the Mystery Coupon, also known as the Penny Item, as a competitive response to other supermarkets in some of our operating areas. After much research and consideration, we found that we can pass along greater savings to a wider range of customers through other promotional offerings and pricing strategies.
In addition to experiencing our clean stores, friendly service, quality products, and extensive selection - customers will be offered great deals including an expanded number of BOGOs, easily accessible digital coupons available online or on your mobile device, lower everyday pricing, and exciting promotions in partnership with our suppliers. We are confident that you will find a better overall shopping experience and terrific savings at your local Publix. We look forward to serving you and making shopping a pleasure.
The last Mystery Coupon will be 12/14 or 12/15 2016 pending the day your ad breaks. This makes me a little sad as while I didn't always get the item...I did like it when it was the things that I really liked (toilet paper, water, etc...) Feel free to share your thoughts in the comments.