As promised, MATE 1.16 will bring more GTK+3 porting. This time, it will be engrampa, mate-notifications-daemon, mate-session-manager, mate-polkit, and mate-terminal. Due to short development cycle, maybe not all of the TODO entries listed in the ROADMAP will be fixed in this cycle. Long term goal is still to have all code ported to GTK+3. It's still a long road ahead due to long list of showstopper bugs, but once we completed that milestone, we can have some new interesting features, such as HiDPI + Wayland support and libunique replacement with GtkApplication.
Although some Linux distribution have offered full GTK+3 only build (such as Fedora), for next 1.16, i will probably keep providing a mix build of GTK+2/3 due to showstoppers mentioned above. Full support will be given to Slackware 14.2 only and not for earlier releases. There will be 3 new deps added in MATE 1.16 which is not yet available in Slackware 14.2: