The whole "platform authenticator" thing enabling passkeys came later. Extending the spec that way was easy: a platform authenticator works just like a hardware authenticator, it just uses a different channel for communication.
The spec the providers built upon just wasn't designed for software authenticators that allow moving around credentials. The original spec assumed credentials are stored in a non-extractable manner in HSMs.
Edit: thinking about it, platform authenticators may have been in there pretty early, but under the assumption of also using an HSM and not allowing extraction of credentials. Providers compromised security for usability, removed the HSM and made passkeys synchronizable – the spec had to adapt.
Are there any examples of any widely-used sites that are enforcing attestation?
- Cloudflare had a "captcha" POC called "Cryptographic Attestation of Personhood" where you need to use a FIDO-approved token. It's reusing U2F just for the attestation part only. I don't think it ever go to production as most people don't have a token (but perhaps in the future hardware-locked passkey may serve as one...)
- Okta do have an option to enforce attestation. By default it is off, but in my Okta production I can limit the list to FIDO-approved vendor only, or to even a subset of them. They also have a beta feature flag for blocking Passkeys but allowing physical keys (which they do not guarantee success)
Are there any widely-used sites that actually enforce attestation?
Basically put it there for nerds and IT where the device owner wants that extra security and coordinates with (or is) the service provider to set it up. For everyday use, it should be unavailable so that it's not used for lockin.