Earlier quoted context omitted.
Are you aware of any age verification systems that do not have this property? (This includes being robust against law enforcement action, legal or otherwise.)
Like many mention in other comments on this post, it's possible to implement using ZKPs. There are likely other methods that would be effective without compromising privacy. None of them are part of the Age Verification discussion because kids are not the actual point of Age Verification. When I say "if we have to know you're not a kid, we have to know who you are" I'm not stating an actual truth , but the argument a…
This would be far easier to implement for websites too. You'd just have a single, unauthenticated API endpoint which, given a code, tells you if the code is valid (and marks it as used). Integrating with such an API is about 1 day of work for a competent dev. Even open, non-profit platforms like Mastodon could easily implement such a mechanism.
Scratch cards wouldn't have to be the only way of getting such codes. THe vast majority of people could just generate them in their banking app or whatever (which would still be far more privacy friendly than the current ID verification mechanisms).