I am just a dog owner like everybody else with zero expertise in dog nutrition. That said, I did spend quite a bit of time researching dog food when I bought my latest pair of young gun dogs. I did a lot of reading, talked with several different breeders on what they fed and why. I talked with people who ran their dogs in hunt tests, field trials etc. and while the brand sometimes differed the one consistent was most recommended feeding 30/20. A few fed 24/20 to their kennel. Clearly either will work.
One night I found a research study that stated that hunting dogs fed dog food with 30% protein had fewer soft tissue injuries than those that were fed lower protein levels. Another study I read said that dogs fed 20% fat found significantly more birds than dogs fed lower fat food. So for me, what articles I read supported what most breeders and competitors were telling me to feed.
I also know some change their dog food over the course of the year from lower fat and protein to higher depending on time of year. That wasn't recommended to me.I found an article on dog food that quoted a nutritionist at Purina:
"It's not advantageous to switch to a dog food lower in protein or carbohydrates during the summer months. Purina Research Nutritionist Brian Zanghi, PhD, explains, 'Switching to a maintenance diet in the off-season is like metabolically detraining your dog.'
Rather, Zanghi advises feeding a high-protein/high-fat performance dog food year-round, such as Purina Pro Plan SPORT Performance 30/20 Formula. 'It's best to reduce the portion fed in the off-season and monitor body condition to help keep your dog healthy and fit.'
The negatives I found on feeding 30/20 were focused around weight(a fat dog) or dogs with existing liver issues or food allergies. I have not found any data or research that indicates that 30/20 or similar food damages a dogs liver or any other organs.
I don't know that there is a right or wrong answer to dog food. Every dog is different and different breeds have different considerations. People 50 years ago had great dogs and I would guess that none were fed a 30/20 diet. In the end, I think that people should feed what they think is best. Find something that works and stick with it if you can.
After 40 years of owning dogs, I have never found changing dog food frequently to be beneficial. The only thing I didn't anticipate when I got my current pups was inflation and the jump in cost of dog food.