[lknuth]

Speaking of passkeys, could they be used to authenticate to a local application - say for unlocking a password vault (perhaps through a Yubikey)?

[dist-epoch]

Probably not.

But YubiKey supports multiple protocols, one of them surely could work for your use case.

[shortsunblack]

Yes. HMAC extension allows for this use case.

[angry_octet]

They could, it's just an API.