A chicken supreme is really nothing fancier than a boneless, skinless chicken breast. Really! That's all! Sometimes it will come with the wing segment still attached, but according to Julia Child, this would technically be called a cotelette instead of a supreme.
We have a feeling that the specification for chicken supremes in recipes is a holdover from the time when most people bought whole chickens instead of chicken parts. Calling for a chicken supreme would tell the cook exactly how to portion the chicken and prepare the breast meat for cooking.
In any case, if you ever see chicken supreme called for in a recipe, rest assured that you can substitute regular boneless, skinless chicken breasts.
It might be interesting to know that there is also a dish called a "Chicken Supreme." This is usually chicken breasts poached in milk or cream, often with bacon and swiss cheese. We're fairly sure we remember some variation this from childhood!
Related: Word of Mouth: Spatchcock Chicken