Since nvidia drivers do not properly implement implicit sync, this protocol not existing is the root cause of flickering with nvidia graphics on Wayland. This MR being merged means that Wayland might finally be usable with nvidia graphics with the next driver release.
In addition to what others said a recent Nvidia driver update also added a workaround to reduce out of order frames without explicit sync. Ime it just made it so that resizing a flickering window makes it stop.