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...
Does it also restore the content of unsaved files of the application? If not, I'll prefer systemctl hibernate. I wonder, what this new feature is for. Gnome had it in the past, MacOS has it, but I don't see what the use case is.
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.