The Forza Motorsport 6 NASCAR Expansion was released on May 17th, 2016, and includes a new track as well as a number of new cars.
Check out the full Forza Motorsport 6 car list, here. FM6 officially reached end of life status on September 15th, 2019. So you can’t buy it as a digital download, along with any DLC. But you can still play offline by buying a physical copy from Amazon or eBay.