I've tested it on both an S650 Ecoboost and an S650 Mustang GT.
It didn't seem to do anything when I unplugged, no dash lights like on S550. If you run a code scanner, it does detect it's unplugged, but there seems to be no noticeable difference in the car's braking, ABS behavior, handling, burnouts, donuts, etc.
According to a wiring diagram someone posted, it goes to the wheel speed sensor, which theoretically should disable AdvanceTrac (ESC) and ABS when unplugged. However, I've simply found that not to be the case. Here's to hoping a benevolent Ford engineer/tech may come on the forum and explain for us sometime...