It's still a largely unsolved problem, unfortunately. Enabling private key backup comes with a suite of nasty problems like what it means for device attestation

It's still a largely unsolved problem, unfortunately. Enabling private key backup comes with a suite of nasty problems like what it means for device attestation and how to guarantee that a key hasn't been cloned in transit. Our best recommendation right now is to have a backup key, but it still means you have to register it everywhere in advance and then go to each site to revoke the lost key. At least Web Authentication platform credentials should let you have multiple authenticators without having to buy an extra YubiKey.