Thanks for the answers, @nik.baleca 
They confirmed my initial hunch that Auth0 does not support this kind of use case.
If we decide to implement passkeys without allowing them to be transferable, there is another issue.
As I mentioned earlier, we currently generate the PublicKeyCredentialCreationOptions on our side. At that point, we don’t know the Auth0 user’s user_handle.
It feels a bit unnecessary to create a dummy authentication method just to obtain Auth0’s internal user_handle? Here, my assumption is that the user identity ID should match the user_handle in options data.
It would be nice if there were an endpoint that could return the PublicKeyCredentialCreationOptions, something along the lines of:
“We want to start passkey creation for user X. Generate the options using these values for the authenticator selection criteria etc: resident key, user verification, and hints.”
There might already be an endpoint that supports this, but I may have simply missed it.