You want something cheap, inert, resilient, and unlikely to be repurposed for any other use.

QR codes are one of the first sort of barcodes to come to my mind for storage; but I'm reminded of how poorly they encode text compared to analog characters on a printed page. The printed letters are also a technology that is already known and self documenting. If you can understand the content there's no intermediate representation.

Mask ROMs are engraved rocks.

The pattern of entanglements on the surface of a black hole.