I believe they're standard. The only Mustangs that don't have the blind spot mirrors have the electronic blind spot monitoring system, in which case a light on your mirror turns on if there is a car in that blind spot.
Okay, I checked and in the dealer sourcebook it says its standard across the board so I'm guessing you're right and they are only not on the ones with the blind spot assist.
Yep, the blind spot mirrors are standard on all models. BLIS (Blind Spot Information System) is offered as optional on both the GT Premium and EcoBoost Premium.