Comment 1 for bug 1712390

(Copied from bug comments:)

Ubuntu is going to ship Unity-7 to make the transition easier. Also Mate is using hud and unity-gtk-module, xfce is using indicators and all (and hence various unity dependency) and xfce also prefer titlebar instead of headerbar. These patches made carefully so that it doesn't affect gnome-shell anyway.

Gnome-apps designed was designed that way only for gnome. But they also made a door if a distro wants to patch it back. (I personally, like headerbar, but I can't impose my personal choice over everyone else.)