Yep, introduced in Android 4.2, disappeared with Android 5.

Google giveth and Google taketh away.

Yeah, they were definitely removed at some point.

I'm wondering what reasons for opting-out could be

Does it still interact funny with the phone screen lock? When it first came out, I vaguely remember something about disabling the phone lock, and using KLCK's built in phone "lock"? It was basically a skin over the home screen. This was years ago, so the details have been forgotten.

Plus finger print is so fast I barely see the screen anyways. Although having improved notification info would be nice.

I love KLWP, but the security implications of KWLC were concerning at the time.