You should be able to access it. Hold down on the gear at the top right for a few seconds and then let go.. it'll bring it back. As you can see I have it.
I have tried holding down the settings gear and nothing happens. I'm trying to enable the battery percentage in the notification bar. After 7.1.1 I can't access system UI to enable that feature.
Interesting. I hadn't noticed that the System User Interface had vanished from my settings, but by golly it was gone! I had to re-enable it by holding down the gear, etc. Had to do it a couple of times before it worked, but it's back now.