Apple’s iOS 11.3 may use iCloud as a single sign-on for websites
There’s also an element in the beta which uses the camera app to scan for a QR code and ask for your Apple ID, although there’s even less known about how that would work.
It’s not certain when this feature will be ready, assuming it survives. While it could show up in the finished version of iOS 11.3, Apple has been known to push back or cut features. AirPlay 2 was originally expected to show up in an iOS 11.2 release, but Apple pulled it and eventually moved it to 11.3. Mind you, it’s easy to see this showing up soon. The company already has a single sign-on option for Apple TV users with a similar goal: take the headaches out of logging in to all the services you use.