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You can use KeePassXC for passkeys. It will generate completely unidentifiable public keys, and save the the private keys to a portable KDBX file.

It's unfortunate that passkeys have been such a disaster. Attestation should never have been part of the spec, it should never have been presented as a replacement for hardware U2F keys, and a private key file format should have been defined on day 1. But there is useful functionality buried under all the noise and confusion.