Attic gable end vents still work.

The peak of the roof shown here has a metal roof ridge vent that just shows from under the snow. That ridge vent is covered with snow and not venting the attic, but the gable vent is clear and still exhausting the warm attic air.
The snow coverage here is not that bad as the snow was the light and fluffy variety and was blown into drifts by the high winds. A normal winter here can cover the roof with eighteen inches of packed snow and it will stay there for a month or more.
The gable wall vent in the picture was installed by the carpenters who built the addition. They were a crew of older brothers who have worked in this area for over forty years, and that is a ventilation design that works very well for this part of the country.