I have the Heated Windshield option in my EQS 450 4Matic SUV. It has not been quite cold enough here in Philadelphia for this option to be activated. My understanding is that it will not necessarily be useful in clearing a fogged window since there are no controls to turn this option on/off. I believe the Heated Windshield will be turned on in conjunction with the cabin preheat feature. So theoretically, turning on the cabin preheat 20 or 30 minutes before departure should clear any ice from the windshield. It's not clear to me if the Heated Windshield remains active while driving. If it does, then it may very well prevent the windshield from fogging in the first place.
I expect to be able to observe the operation of this feature in the next couple of weeks as temperatures drop below freezing.