Yes, it looks like it’s been around for a couple of months, after a somewhat quiet announcement. It’s the first I had heard of it though.

There was a WWDC session last year about Passkey:

https://developer.apple.com/videos/play/wwdc2021/10106/

Since Fall 2019 actually! (at least that's when I first used it)

I think you are confusing this with U2F support using the secure element, which has been added much earlier (I have also been using it for years already). The difference between that and Passkey is that Passkey syncs private keys between Apple devices through iCloud Keychain. The older U2F support is per-device.

It is, but annoyingly you need to enable it via Xcode, so if you only own idevices you can't turn it on. (Unless you mean just normally adding an iPhone as a security key & using Face ID to authenticate it, which is just how Apple implemented FIDO2 and isn't synced like Passkeys)

Is there a list of web properties that support passkeys?

I think it's supposed to work on websites that support Webauthn/FIDO2. For example, I have been using experimental Passkey support in Safari as a second factor for GitHub.

Yup, I think the bigger announcement today is around it becoming available on non-Apple devices.