Feeding your chickens after a SHTF event is doable and not as difficult as you may think. I will list here a bunch of food option for your chickens without getting into a bunch of needless dialog. Obviously the harsher your winter the more difficult the job. Enough about all that. On to feeding your chickens.
Foraging on their own. Much of the year or even all year. Some breeds will not need any additional food.
Extra grain if you are lucky enough to have it.
Sprouting your grain is even better than unsprouted grain.
Kitchen leftovers, but plant and meat.
Seeds. Lots of weed seeds can be easily gathered and given to the chickens to see if they like it.
Worms, chickens lover worms. So compost piles are great.
Grub and maggots. Rotting flesh held off the ground. Maggots fall to the ground chicken have a party.
Cricket. Crickets can be raised purposely for your chickens.
All sorts of weeds and grass. Part of free-range diet.
Vegetable that are extra or have issues that make you not want to eat them.
Fish. Either the whole fish are what's left after you clean it for yourself.
Hay, dried grass.
Bugs, bugs and more bugs!
Acorns. Not whole, but broken/crushed up when being fed to the chickens. Store them in their shells and they last much longer than shelled acorns.
All manner of nuts, pecans, walnuts, etc.!
Ground up egg shells. Great source of minerals for the birds.
Many many berries.