and why i saw many many landscapes shots in that diffraction f-stops?
If you need the DOF then you stop down as far as you must to get it. Diffraction softening will always be a secondary consideration after getting the scene in focus.
But 135mm lenses are long, and not often used in the sense of a landscape scene where there are subjects both near and far from the camera. In the cases where such a shot is desired then the lens will need to be stopped down.
So final point - if all you shoot is landscapes and you plan to shoot the 135L always stopped down past f/11.....IMO you can easily get the same results from much cheaper lenses.
When you buy a telephoto L prime you are usually paying for a fast lens that performs very well wide open. By f/11 an L prime and a cheapo zoom are going to be pretty darn close in performance
