When I try to add an ecdsa-sk or ed25519-sk key generated using my hardware U2F/FIDO security key to ~/.ssh/authorized_keys.d/, and run the update-ssh-keys command, I get the following message:
$ update-ssh-keys
warning: failed to parse public key "sk-ssh-ed25519@openssh.com AAAAGnNrLXNzaC1l...AAAABHNzaDo= yubikey": unsupported keytype: sk-ssh-ed25519@openssh.com, omitting from authorized_keys
warning: failed to parse public key "sk-ecdsa-sha2-nistp256@openssh.com AAAAInNr...AAAAEc3NoOg== titan": unsupported keytype: sk-ecdsa-sha2-nistp256@openssh.com, omitting from authorized_keys

When I manually add the public key to the ~/.ssh/authorized_keys file, the authentication works fine, but of course this does not persist between reboots. Security key-based authentication has been supported in OpenSSH since version 8.2p1. It would be nice if there was support for this in the update-ssh-key tool, since the current stable version of Flatcar Linux runs OpenSSH 8.7p1.