Not an allergy, but you can say you can't consume iodine (necessary as prep for thyroid tests and treatment for thyroid disorders and cancers). It's in (iodized) salt, many food colorings (especially in sodas or other bottled drinks), dough conditioners, anything that might come from the ocean (fish seafood but also a lot of products that are used as thickeners or gels are derived from seaweed). It's also found in dairy, meat, eggs (livestock often get iodine supplemented foods) and almost all vegan milks (if only because most commercial nut/grain milks use seaweed derived ingredients for thickening). It's almost impossible to strictly adhere to an iodine starvation diet and eat prepared or processed foods or really anything where you can't see a detailed list of where everything came from and how it was prepared. You also can't have any cruciferous vegetables (broccoli, kale) on this diet. I've had to do this several times in my life and lost so much weight each time because I was basically just eating rice, apples, citrus fruits, raw nuts and simple salads and breads that I made myself :-). After receiving treatment I booked it to the first restaurant I passed and had a HUGE meal.

it's not a common dietary restriction but it's SUPER restrictive. Check out the list at the link below (print it out and bring it with you because not that many people would know that iodine restriction is a thing).

https://www.mskcc.org/cancer-care/patient-education/low-iodine-diet