First I'll give the formal position. All treats should not make up more than 10% of the overall diet. This includes kitchen wastes, greens from the garden, insects, and scratch. The theory is that the layer ration is a balanced food and contains everything they need. If you feed too much other stuff, especially stuff they prefer to eat over their layer ration, their diet gets out of balance. One rule of thumb is that they should be able to clean up all treats in a 10 to 20 minute period.
Another theory is that too much scratch will cause them to get fat, especially around their vent area and get fat on their internal organs. The extra fat around the vent can supposedly cause them to get egg bound or prolapse. The fat on their internal organs can cause something caused fatty liver syndrome. Fatty liver syndrome does kill chickens. You can google it to confirm if you don't believe it.
I've also heard that if you feed scratch or just straight corn just before they roost, that it is high energy and will help keep them warm at night. I'm guessing that is why you want to feed yours scratch in the winter.
I'm not a veterinarian and I have not spent my life studying chicken nutrition like some of the people at the Universiies associated with the extension services, usually the state land grant university. From my reading, both those first two theories (balanced nutrition and fatty liver syndrome) theories come from people who have studied chickens, usually in conjunction with the big commercial poultry operations. I'm not sure where the "keep them warm in winter" comes from but it could certainly be from a similar source. The theory makes sense to me. The commercial operations are the ones that bring tax revenue into the states, provide jobs, and provide funds to the universities to pay for the research, so I can certainly understand that emphasis. Thus you have to take whatever you read from the extension services or universities with that in mind, although some extension services do specifically comment on small hobby flocks like ours.
I do not feed scratch at any time. Mine get layer plus whatever kitchen scraps, greens, and Japanese beetles in season that I feed them. I don't know where you are located or how cold it gets (you might want to modify your profile to show that information. It does sometimes help in the conversation on here), but I have experienced (+) 9 degrees Fahrenheit already this winter and mine had no ill effects without the extra energy from scratch. I do not provide heat either, just a well ventilated, draft-free coop.
On the diet I mentioned, I have noticed when I process mine that they do have a heavy glob of fat in the vent area and they do have fat on their gizzard and liver. It seems to go by breed with the Australorp the ones with the most fat although they all have it. Maybe that is why Australorps are supposed to be better winter layers than some nother breeds. I really don't know about that. The ones with a lot of fat do not look fat while they are living and appear as active as the others. I have had one Australorp die with no apparent injuries and no apparent cause. I am not saying it was fatty liver syndrome, but I believe it is certainly possible. This is without a supposedly fattening diet.
Many people feed a lot of scratch and have no apparent ill effects. Some people seem to do things by the theories and still have problems. I'm not going to try to tell you what to do, just try to provide some information from my perspective and let you make up your own mind.
Good luck however you decide.