Comment by jasomill

With the exception of general-purpose authentication services like Microsoft Entra ID where organizations can set pretty much arbitrary authentication requirements, I've yet to encounter a site where merely setting up a passkey disables other forms of authentication.

The benefit of adding additional authentication methods without disabling others is limited, of course, but it can still be useful where, e.g., you're comfortable logging in using a password manager on your own devices but want to use an iPhone or YubiKey to log in from public computers.