When using a passkey as the only factor, it gets complicated, but I think that will be the future: Email and SMS as fallbacks with additional factors like risk-based location checks (or local storage hints) to have a viable recovery mechanism parallel to passkeys. I think Github has a nice approach that we use as a role model: Collect as many fallbacks as possible in a convenient manner. Adding passkeys on any new device and regularly reminding people to check their fallbacks. That's still way more secure than passwords only, where the fallback is practically always a form of email OTP. Also social logins from Google or Apple can act as a fallback as they are (mostly) 2FA.