I like paper documents for this very reason. It's very hard to steal everyone's documents when they weight about the same as a train.
But it’s also very easy to lose all of them in a fire or flood. Different tradeoffs.
"Fireproof file rooms and cabinets in the 1920s were crucial for protecting business and government records during the rapid expansion of the industrial era. The era saw a massive shift from flammable wooden office furniture to robust, steel-based storage designed to resist both fire and water damage."
That's a Google AI summary - but I've been in a fair number of buildings with such rooms. Thick concrete walls, heavy steel fire doors, no other openings, nothing but steel file cabinets in 'em, sealed electric light fixtures that look like they belong in a powder magazine (where one spark could kill everyone) - it's really simple tech.
And "high ground" was a reliable flood protection tech several centuries before that.