as in the video currently Free Transform Scale Tool has either free scaling from a point or proportionally scaling by holding Ctrl. It could be useful if we also add Mouse Direction Based Constrained Scaling, which is scaling either on X direction or Y direction Scaling but NOT affecting the other. keyboard shortcut could be Shift+ Drag because, why not? it is empty.
WHEN THIS IS USEFUL?
If the Bounding Box is out of working area you have to zoom out to scale in one direction. By this way you don't have to