Same for my two cats, they are on non-chicken diets (rabbit-based and I think duck-based) and their vomiting, diarrhea, and itchiness have all been greatly reduced or outright eliminated.

My partner (a practicing small animal veterinarian) sorted them out a few years ago, and also confirms that chicken allergies are super common. They were fine for 5 or 6 years of their lives, then they just were unable to handle the chicken. Figuring out which of the alternatives they liked enough to consistently eat was the toughest part, lol, they're super picky.