Implement the XDG session management protocol, in order to allow clients to ask the compositor to save window state for them. This works via creating a xdg_session_v1 object...
Gnome like to get things perfect before they make it default. It's what makes Gnome pretty stable, even if it does mean power users have to type in a command to expose the setting in the meantime.
Stability for the end user is very good (probably even one of the best and definitely many times better than KDE) but stability for developers is not good because things often change or get deprecated which breaks the apps and the extensions they make.