> xdg-session-management for being able to save and restore window positions > is still not merged, so there is no standard way to implement this in Wayland

For me, this is a real reason not to want to be forced to use Wayland. I'm sure the implementation of Wayland in xfce is a long time off, and the dropping of Xwindows even further off, so hopefully this problem will have been solved by then.