Why not?
When will they be made available?
Personal opinion: they seem a completely obvious addition to make, and let's be frank here: they should have been added along with the introduction of Auto Layout some years back. Along with many other tools and and API that would have made Auto Layout much more friendly and useful.
To me, the lack of NS/UILayoutGuide support in Xcode is symptomatic of Apple's approach to layout in general. It's taken 9 iterations of iOS just for us to get a clone of Android's LinearLayout in the form of NS/UIStackView, and we still have major difficulties with the practical aspects tasks of laying out Auto Layout constrained elements in Interface Builder, and mixing them with Size Classes. Not to mention the difficulty curve of Auto Layout in general, and the verbose API.