Heating would have no real impact, ICE cars are heated using the heat their engines naturally produce.

Winter tyres though and winter fuel mixes? Those definitely have an effect.

Heating shouldn't take any fuel efficiency in a petrol-powered car since the heat is (usually) just a by-product of the engine's inefficiencies.

Letting the vehicle idle in the driveway while the windshield defrosts will decrease fuel mileage, because the engine runs for longer.

It's not really the fault of "heating" that you're idling your car unnecessarily. I'm definitely not idling my car to defrost my windshield, as I'll most likely pre-heat my car, or if I don't, I'll scrape the frost off.

The reason I idle before driving is that even after I scrape the ice off, the wipers don't seem to be as effective until the window is warmer, and it's blurry and gets foggy. I try and scrape, wipe and antifreeze as much as possible, but I still find I can't see as well until the windows warm up a bit.

I feel like we’re losing context here. My comment wasn’t in reference to what I do, or what you do. It was in reference to what the root comment said they did.

The person in the root comment proposed that their practice of letting their ICE car warm up to defrost in the winter may lead to decreased gas mileage. This is correct; it will.