Have you considered a raw or home cooked diet? Making one from scratch means you control the ingredients that go into the food. The Catinfo.org recipe is a good one to start but with a novel protein instead of chicken. A semi-homemade way to make a raw or cooked diet is to use a pre-mix such as Alnutrin with the meat of your choice. Hare-Today.com has a ton of novel proteins you can try for either a raw or cooked diet.
A commercial raw diet might be an easier way to start. Freeze dried and air dried raw are usually more acceptable to cats than frozen raw.
Is it just proteins that your cat is allergic to? All sorts of things in commercial cat food can cause allergies in cats: oils, veggies, legumes, etc. Leroy is allergic to goat milk and mussels

Some time ago on a different cat board there was a person whose cat was allergic to pretty much everything in cat food. There was maybe two or three foods the cat could eat with minimal issues. I think the cat had some immune compromised health status but I don't recall. The person had a spreadsheet with a ton of canned cat foods and their ingredients and used it to track which foods the cat could and couldn't eat. I have a copy of it but it needs some updating before I can post it for sharing.
A bland-ish food is often helpful. Rayne has a kangaroo and a rabbit canned food you could try.
https://raynenutrition.com/
Probiotics might be helpful as Wendy mentioned. Another good web site is
http://www.rawfeedingforibdcats.org/probiotics-for-cats---why-and-which-ones.html
For diarrhea, there might be info here
https://www.foodfurlife.com/my-cat-has-diarrhea---what-do-i-do.html#/ Looks like S. boulardii with MOS is very helpful.