It was a featured added to LR CC for subscribers only.
I understand. It is a great tool used by simply drawing lines on what should be vertical/horizontal. I use it a lot. Since the release of LR6 Adobe continues to provide new camera support but new features are only added to the CC version.
List of features in LR CC not found in LR6
LR Mobile sync (other than LR Mobile trial)
LR Mobile Webspace
Dehaze tool (Effects)
Boundary warp tool (available with panorama)
Guided Upright (transform)
Selective adjustments with dehaze, whites & blacks (available with the graduated filter, radial filter and brush tools)
Reference View with active window
Filter for Snapshots or as criterion for Smart Collection
LR mobile sync recovery
As at March 2017 – release of LR 6.9/LRCC 2015.9
It is indeed a great tool!
My own feeling is that it should be part of LR6 stand alone updates since it is not exactly a new tool, it is more of an improved tool.
Adobe has its own marketing arguments, I guess...