My guess is that it would be rather difficult for even nation-state hackers to demolish the US IRS mainframe-based Individual Master File https://en.wikipedia.org/wiki/Individual_Master_File which is still mostly written in Cobol and assembler.
There is something to be said for the safety of outdated practices.
Sure, paper is a safe storage medium. But how would one restore from paper backups? Put that mountain of paper through scanners?
Doable, yes. But if it takes extreme amount of time and/or people power, then how useful are those backups, really?
You would start by spreading rumors of impending coordinated multi prong attack at all storage facilities, simultaneously your inside man proposes relocating archives to a safe central location or just sending them all to a recovery facility.