A real innovation from the Bitcoin world! There are several physical password store systems that they have suggested for this kind of use case. The simplest is basically using a nail to punch out a password onto a piece of sheet metal.

Articles such as https://blog.lopp.net/metal-bitcoin-seed-storage-stress-test... will help you pick among the various seed stores out there.

And so we return to our programming-roots with punchcards. :p

Additionally hardware wallets which can use a seed to generate huge variety of keys.

Including AGE keys (so you can encrypt arbitrary data), SSH keys, FIDO2 and passkeys.

Additionally you might want to store a hardware wallet in a deposit box instead of the seed (if you trust the security model).

Just make sure that the metal you use has a high enough melting point.

Do people usually find big pools of metal on the ground in burned houses, or is everyday metal fine?

Especially inside a fireproof safe.

Wouldn't trust aluminium, solder, Wood's metal, gallium, or mercury, but apart from that...

Maybe a clay tablet (assuming it's safe from water)?

Tungsten, perhaps.