I hate decyphering hieroglyphics. I propose that the unicode for "I have peanut allergies" should be the text string "I have peanut allergies."
That works well for 1-2 billion people and not so well for the remaining 5-6 billion. While we're working on that universal language, a few universal "hieroglyphics" are useful and there's no law against writing elevator next to the elevator sign. Like say these, these, these or these.
That said, allergens may be useful for store products but that's usually half the markings on a restaurant menu which typically can be stuff like vegetarian, vegan, hot, garlic and so on. And for many complete dishes many will contain lots of allergens, it's probably easier to use a negative marking like these. I don't quite see what existing use case these symbols are supposed to cover, yes it could be added to the ingredients list but you need to solve other issues like how do you prominently say no allergens and not unmarked?