I mean, I am not sure if you can be held liable if someone who knows they have allergies doesn't ask about allergens in whatever you're offering but I yhink that depends on why yhey wouldn't ask.
Like if they simply forget to ask, that might be negligent on their part, unless
whatever you're offering is not something they nor anyone else would reasonably expect to contain food allergens they are allergic to.
Like, I know of resteraunts that got in trouble for reusing the frying oil used to fry shrimp to also make french fries. The customer with the shrimp allergy orders the fries reasonably expecting the fries to be safe for them to eat, and then has an allergic reaction to the fries because of all the residual shrimp proteins the resteraunt did not warn them about.