I literally just enabled this 1 hour ago, for unrelated reasons.

However, for those reading along, initially the 1Password web interface for my account only offered the choice of setting up a TOTP authenticator. I completed that, and still saw no option for enabling a FIDO/YubiKey device. I then went into the 2FA settings for my account, toggled the option for YubiKey support off and then on again, and returned to the 2FA settings page. Only then did I see the option to enable a YubiKey.

I was then able to add my YubiKey and I can confirm that it's working with my 1Password account as a 2FA source.