I've spent hours looking and could never find anything about 14 hours of light causing cancer. I did find information on one study that artificially keeping them in darkness, I think it was less than 8 hours of light a day, helped treat existing ovarian cancer. That's not the same thing as saying lighting causes cancer. If anyone ever runs across some actual studies, I'd really like to read them.
I provide supplemental light to our parrot, because they are healthier with a 12 hour day of good lighting, year round. That means I also make sure he is getting enough sleep on the long days of summer, too.
For the chickens, we've given them supplemental light in the past with no problem. That doesn't mean it's the best way to do it. I like to keep up with new ways of doing things, too. It's just that sometimes, I've been doing things a particular way for a few decades and I need an actual reason to change.
One of the advantages of more light in the bitter cold, is that they have more time to eat and fuel their bodies. A really long night in the bitter cold is not easy on birds, either. It's what kills a lot of the wild birds in winter, from what I've read.
I have a lot of different feelings about winter lighting and I just keeping reading, to see what others have to say.