They are updating it, but keeping the original style as much as possible. The forked apps, like the file manager (caja), archive manager (engrampa), text editor

They are updating it, but keeping the original style as much as possible. The forked apps, like the file manager (caja), archive manager (engrampa), text editor (pluma) etc. still have menubars and so on, but, for example, it is using dconf/gsettings and MATE in Ubuntu MATE 16.10 is all GTK+3. They've also integrated features for setting panel layouts, and there is one ("Mutiny") that mimics Unity. They're even working on a HUD-like feature.