For example, a super-hacker writes a virus that crawls electronic databases and purges all data on supers (or subtly adjusts it so the biometrics no longer match the secret identities).
This has no effect on physical records like photographs, but when was the last time you took a picture with anything other than a phone? (Alternatively, you may think Clark and Superman are the same person, but there's no facial recognition match. It's proof that it's just a superficial resemblance!)
Doug