For the client-side, the spec is comprehensive in allowing the authenticator to decide whether backups are allowed. In this case it's iOS not exposing that to you as a user. I get why you'd want this, but trusting Apple to store your single-device passkeys for high-stakes credentials but not trusting them for syncing them is somewhat of a very specific threat model I'd say, and definitely not in Apple's own interest to support, to your detriment.

RP-side, it's true that RPs can't opt out of credential syncing, but I think that would be weak at best, as the authenticator can do what it wants. The RP can use attestation and the DPK extension to effectively bind authentications to the same originating device.