Goldfish and koi are very hardy, they can survive in the winter as long as the pond doesn't freeze solid. My pond is nearly 5 feet deep, it's half above ground and half below ground, so sometimes the top freezes over in the winter, but it never freezes below the surface.
If you don't keep a hole in the ice for gas exchange, your fish might all die. We use a pond de-icer (keeps only a small hole open.) I have to give about 100 fantail goldfish away to the pet store every year from a 5x7, 3 foot deep pond. Do not feed goldfish when the water is below 50 degrees, and it's best to feed a special cool-water food during fall & spring in case there is a sudden cold snap. Their digestion slows remarkably when cold, and gas can build up in their intestines and kill them if their gut is full. The cool-water food clears their system quicker than the normal warm-weather formulas.