My cat is pretty allergic. She has had GI allergies to beef since we rescued her but recently developed dermatitis and now eosinophilic plaque from allergies.
We were recommended to put her on a hypoallergenic diet (she's also had a steroid injection) but she refuses the biscuits and I'm 90% sure is finding another cat's food, she's been going out for longer periods, then coming back in and vomiting as if she's had beef.
Aside from repeated steroids and a hypoallergenic diet, the vet hasn't suggested much. I wondered if anyone else has found success for an allergic cat in a high-cat area which haven't involved keeping their cat inside to ensure she only gets specific food (she was a stray before we adopted her and would struggle hugely without her cat-flap access).
We're not wealthy so personalised immunotherapy is likely not affordable, but we love her hugely and she has good insurance so normal veterinary options are readily available. Cat tax included!