Raw soy has trypsin inhibitors that can affect growth. There may be other consequences, the jury's still out on them. Soy that has been cooked/boiled has most of this neutralized, and fermented soy has this more neutralized. Humans and poultry can be affected, cattle are not; I guess if you have extra stomachs, you get a pass on this one.
I doubt that there is anything magic about feeding soy beans. It's more likely that with non-caged flocks, there's less insect protein to be had during the winter months, so any feeding solution that provides the right amount of protein is what's needed. It probably works like magic if you're only feeding corn and your birds are poorly nurished during winter months as a result; I'd guess that would be the origin of the old' timer's tales. I doubt it is any more useful than any other proper diet with proper protein levels, though.
You might also have a flock that's meeting some of its calcium needs from insects, earth, and greens during warmer months, and these could be lacking in the winter months. If you're feeding a "layer" diet, you're probably feeding enough calcium. If you have concerns that you are not, put out some free-choice oyster shell. I'd recomend putting this in a separate feeder, and not mixed in with the regular feed. You can actually feed too much calcium, and end up with rough-shelled eggs, or with eggs that can't be pipped by hatching chicks because the shells can get too thick. If the shells on the eggs you are getting are looking good, then adding more calcium is not going to get you more eggs.
