Taking a minute to appreciate the level of long term thinking required for storing data, to plan for 300-500 years into the future, to be able to withstand all kinds of innovations, and survive basic obsolescence.

What is the longest surviving paper medium?

Dead sea scrolls come to mind, for some values of "survive." And the Book of Kells is in good shape.

Longest surviving paper document (as opposed to parchment) https://en.wikipedia.org/wiki/Missal_of_Silos?wprov=sfti1

Seems like they're pretty lax about their recommendations tbh. XLS is "preferred".