So apparently what happened is that they left the code in, but removed the switch in System UI Turner. So those who were in the beta program, installed preview builds, and enabled the feature, still have it enabled after upgrading to 7.0 final. But if you didn't have it enabled before, there's no way to do so out of the box.

However, there's a third party app that provides access to the switch (seems to not work for some people, or require several tries - YMMV): http://android.wonderhowto.com/how-to/enable-hidden-night-mo...

It's on my 6p which had an updated this morning that I assume was N final (I've been running the betas). It's in the dropdown settings pane. You might need to hit "EDIT" and add it as a tile?

I have updated my 6P to final, and it's still there. You might still need to jump through "experimental feature" hoops to enable it though - I haven't checked, but you had to do so in the last preview build.