You can have warnings for the really common triggers (rape, self-harm, etc) and let the rarer ones come to you. Let’s use allergies as an analogy. If I take a dish to a potluck, and it contains peanuts, I will provide a note saying as much. I am allergic to bananas. It is not a common allergy, so I will let the host know about my allergy before hand so dishes with banana can be labelled, or (if I forget) I will ask before taking from any dish. Someone with a peanut allergy will still have the responsibility for avoiding that food, but in most places it is considered courteous to advise of the most common allergens. It’s the same with trigger warnings/ content warnings.