Then a huge number of foods would have such a mark and sufferers would be no better off. In most cases people only react to a small number of substances, or just one, although some unfortunates do have multiple allergies. So they would be reading the fine print to find out if their allergen was present anyway - which is what they ought to do without the “A”.
It makes more sense to ensure that food is accurately and comprehensively labelled and that sufferers are educated and trained to be aware of what they are eating from an early age. Sufferers need also to habitually ask at restaurants and other situations where labelling on the product is impractical (eg home cooking) and to get accurate answers about the content of what they are offered. As I read the article the victim didn’t ask but assumed what was in the dish.
A facile change to legislation isn’t the answer to every problem.
