[Symbiote]

It's so common in the EU and UK that I think it's probably law, or a good way of complying with the law. It's specific to the restaurant's menu.

Some places will list the allergens directly on the menu, with symbols or footnotes. Less common is a separate menu showing only e.g. lactose free options -- I've seen this at nicer places in the UK.

Places like cafés and fast food where the menu is a blackboard or screen should have a book/file or similar. It is often just printed Word documents in a ringbinder, presumably prepared by the chef.

McDonald's example: https://www.tripadvisor.co.uk/LocationPhotoDirectLink-g18704... (just see the picture)

Ryanair in-flight menu example with symbols: https://www.foodsmatter.com/allergy_intolerance/management_t...

(Might _ua mean Ukraine? Then FWIW, I've seen this system in Romania, Bulgaria and Poland.)