This is something I’ve always been really curious about, since a lot of chain restaurants and other food vendors tout their “secret recipe” as a selling point. KFC’s secret 11 herbs and spices comes to mind as a fairly infamous example. Since they’re secret, I assume they wouldn’t show up on an ingredients list and would just have “herbs and spices” as a placeholder? If a customer is severely allergic to one of the herbs or spices, eats KFC, goes into anaphylactic shock and sustains brain damage from lack of oxygen due to their airways closing up, could the restaurant be liable for the injury?
IANAL, and jurisdictions vary, but presumably the customer would have to have told the minimum wage cashier about their allergy. Otherwise it would be difficult to lay blame on a restaurant.
Getting back to the real world: By now we have a pretty good handle on what foods can trigger severe allergic reactions, and companies obviously aren’t going to want to keep such ingredients a secret. So this isn’t a thing that’s likely to actually come up.