MATE 1.18 was recently announced here: http://mate-desktop.org/blog/2017-03-13-mate-1-18-released/
The migration to GTK3+ has been completed.
The upgrade was put in place.
Great to see MATE 1.18 landing so fast in unstable repo.
This is just a detail but mate-calc was added back in this version and it is missing in the repo.