I can register my Yubikeys on account.google.com (and around the web, e.g., fastmail.com) as passkeys. If you visit the account security page[0] and enable "skip password when possible", then you can log in to Google with only a Yubikey-backed passkey.
If you have old Google creds on your Yubikey, you may have to first remove those creds from your account (because there are older and newer protocol choices, and with the old protocols enabled Google will not support passwordless login).
Multiple yubikeys are required if you would like to have backups; there is no syncing between keys.
If you have old Google creds on your Yubikey, you may have to first remove those creds from your account (because there are older and newer protocol choices, and with the old protocols enabled Google will not support passwordless login).
Multiple yubikeys are required if you would like to have backups; there is no syncing between keys.
For support matrices, see [1].
[0]: https://myaccount.google.com/security
[1]: https://passkeys.dev/device-support/