For ordinary websites none of the existing platforms support WebAuthn attestation AFAIK, because the spec for attestation was not designed with key synchronization in mind, which is a key use case; otherwise, basic features like Keychain synchronization via iCloud would not work and the whole thing would be DOA. Synchronization is widely considered to be a core necessity to match the functionality of existing password managers.

I believe you can enforce attestation using MDM and a custom app as far as I'm aware. But for general people using Safari or whatever, you can't request any kind of device attestation.