The 8 most common allergens as identified by the FD: milk, eggs, fish, Crustacean shellfish, tree nuts, peanuts, wheat, and soybeans. These are called out on all packaged foods, which makes it easier, though I'm OK with some derivatives like soy lecithin. I also avoid yeast (brewer's or baker's) and a few other small things like black pepper.

I've been in remission for long enough that I could probably start eating a lot of these things again, but I have no desire to go through another Crohn's episode, so I'll probably continue to avoid these foods for the next 40-50 years (unless there's a miracle Crohn's cure in the meantime). After that, as I understand, a weakening immune system from old age makes Crohn's symptoms significantly less likely or severe.