"Prebiotics" is a synonym for soluble fiber, which helps food move through the digestive system. Sundays' includes many natural prebiotics such as chicory root and several other fruits and veggies like pumpkin.
"Probiotics" are cultured bacteria that can also aid in digestion in a different way. They're becoming increasingly popular in both human and dog food. While the general concept of healthy gut bacteria is being established, there is little to no evidence-based research on the actual measured effect of specific baceteria cultures. Sundays does not currently use probiotics for 2 reasons: 1) they're typically added to dog foods that are difficult to digest or cause GI issues — and Sundays is one of the most digestible dog foods ever measured (with a digestibility score of over 92% compared to ~75-80% typical of most dog foods); 2) the dose of probiotics required to make a difference is not practical for dog food. Many dog food companies claim to add in probiotics, but there are in such tiny amounts it's unlikely to provide actual benefits.