I’m not sure how it could be illegal. It definitely isn’t a good natural primary key (but then I’m firmly in the “natural PKs are harmful” camp anyway). And it’

I’m not sure how it could be illegal. It definitely isn’t a good natural primary key (but then I’m firmly in the “natural PKs are harmful” camp anyway). And it’s certainly too easy to leak PII by using it as a PK. But I’m struggling to imagine a coherent legal structure which accepts collecting SSN data but mandates limits on how it can be used in a database schema.