So far my cat's food allergies have resulted in GI issues.

They developed over time. One cat ate chicken fine for months, and then she didn't.

Neko had the same issue with wheat in gravy. Fine the first half year, then not so much. Have you tried some novel proteins such as lamb, rabbit, duck, venison?
Buzzy has done well on chicken & turkey all his life, so I'm more likely to go back to that if I have to. (The Fancy Feast/Friskies we're using now has "meat by-products," and I know that can be a mix. Also, I was giving him various flavors, which I've stopped - switched to ONLY chicken/turkey flavors, but I think Friskies might still contain some beef/fish anyways?)
We started insulin & switched him to Fancy Feast on Feb 7th. Hair loss became noticeable on ears Feb 22, so 2 weeks later.
The only real things that started before that point were the insulin, home testing, Neosporin on his ears (which I have now stopped doing & just started putting coconut oil on his ears today, which seems to be soothing), and food switch. He started these antibiotics the PM of the 21st, so seems unlikely it'd be related to that.
The other possibility, of course, is some sort of medical issue/fungal infection/whatever, which I know I've also talked about way too much lol. I only know it doesn't glow under a blacklight & I haven't caught it -- which, last time I had foster kittens w/ ringworm, I caught & showed signs before we even noticed it on them. I see no black build-up or waxy build-up in his ears, he's not losing hair anywhere else BUT his ears, there's no rash or crustiness.
It's obvious his ears are sensitive/bugging him though.
I'm unsure if my other cat is having issues -- she had a little bloody spot on her temple last night, but it appears to be healing, not getting worse, and she has no hair loss. It looked like she had just scratched too hard once.
I vaaaaguely remember Buzzy having a sensitivity to some food before, but I can't remember what happened or what food it was.