First noticed by German site iPhone-ticker.de, the CarKey privacy info can be found in the current iOS 13.5.1 release and the iOS 13.6 beta. There's no word yet on when CarKey will be released, but quite a bit of info about the feature has leaked over the course of the last few months as signs of it have shown up in various versions of iOS 13.
Wallet allows you to add and share car keys for certain vehicles. You may add a car key by signing in to your vehicle manufacturer's app or entering a pairing code in Wallet to claim the vehicle as your own and to pair your device with your vehicle. If successful, your device sends Apple a one-time owner redemption token. Apple uses the redemption token, information about your Apple account and your device, and your location at the time of provisioning (if Location Services is enabled) for fraud prevention purposes.
Digital CarKey keys will be stored in the Wallet app alongside credit cards, rewards cards, gift cards, and other items on iPhone and Apple Watch. These keys will let an iPhone or Apple Watch unlock, lock, and start vehicles equipped with NFC.
Vehicle owners will be able to share a digital CarKey with another person, which is useful in valet situations, if a friend needs to borrow a car, and more. CarKey access is authenticated biometrically using Face ID or Touch ID, though there is an "Express Mode" that vehicle owners can take advantage of.
Apple will be partnering with vehicle manufacturers for CarKey, and it may be a factory installed option similar to CarPlay. CarKey requires NFC to function, so it is something that needs to be implemented by car makers.
Screenshots found in iOS 14 have suggested that BMW may be one of the first manufacturers that will support the CarKey feature. CarKey functionality could be released at any time as the NFC-based Digital Key Release specification that powers it was finalized in May and provided to Car Connectivity Consortium members including Apple.
Given that CarKey is a major feature and iOS 14 is on the horizon and set to be unveiled next week, CarKey could perhaps be included in the iOS 14 update.