Back window deflectors were offered as options on a lot of station wagons in the early 1960s and by the end of the decade were standard features on many (most?) models. There were even fancy versions that were built in to the body–some on the roof, some on the sides where car-top carriers would not interfere with their ability to keep dust off the windows. Our first three wagons did not have them and when we first got a car that did have them I was pretty impressed with the way that they actually worked (since so few gadgets actually seem to do what they are intended).
I have always been mildly surprised that they were not employed on mini-vans or the larger SUVs when the behemoths were first introduced. On the other hand, there are a lot fewer gravel roads than there used to be, so it may simply be a matter that few drivers felt they were worth the bother.