Seems like the practical application for this, given the flow structure in food service sourcing, is an opt-in allergen certification.
Business that pride themselves on whole ingredients/ could answer the questions about what's in their food could seek that status and be listed as allergen safe establishments.
People who need to know where's safe to go could check an app, or a website, or a list.
In practice, the app could have your allergens loaded as a profile and when you pull up one of the certified places, it could show you the menu with unsafe things marked or safe things marked.
It seems likely to me that there would be an unfortunate correlation between establishments that know exactly what is in everything and ones who are precious/protective about their recipes.
So some obfuscating intermediary that still serves that need would be great.