Context: While the 90% figure is made-up, the premise is true in that most of human history has gone unrecorded. History is considered to have begun with the start of proto-writing (earliest being the Jiahu symbols in China circa 6600 BCE) to writing (3200 BCE), and thus the advent of recorded history.
Thankfully we are having some luck in restoring lost content.
https://www.nationalgeographic.com/history/article/herculaneum-scrolls-mount-vesuvius-ai
https://www.theguardian.com/tv-and-radio/2026/mar/13/two-lost-doctor-who-episodes-from-the-1960s-found-and-regenerated