I think Apple announced this iCloud Keychain feature in iOS 8, which allows developers to use your saved ID and passwords directly in apps (as opposed to just browsers). For some unknown reason, virtually no one has implemented it at all, and I think most have forgotten that it even exists.
Wow, really since iOS 8? In the previous release of imgur I'm sure it wasn't there! Yes, it's a great feature, can't understand why the developers don't use it