Maybe Holdo was the first person to think up the kamakizee lightspeed attack. As others have pointed out kamakizee is only useful in certain situations, it makes sense no one would have thought of it until now. They did say she was a brilliant general, and Poe + Hux (both experienced in the military) are confused when she turns the ship around.
That seems highly unlikely. Self-sacrifice is probably older than the discovery of fire, and kamikazee was actually fairly widely used by the Japanese in WWII. If we have invented it many times over throughout our earthly history, I would expect Holdo not to be the first to think of it in the much more advanced GFFA. Considering the energy released on impact is proportionate to the mass times the velocity squared, if hyperspace is really nothing more than traveling very fast in real space and time, as it appears to be in TLJ, then an obstacle travelling at lightspeed would obviously inflict the most damage. A lightspeed torpedo should therefore logically exist. Not having it would be the earthly equivalent of having super sonic planes, whilst inexplicably not having super sonic missiles.
Edit: consider that an object of 1kg travelling at the speed of light would have a the same energy released on impact as a nuclear bomb.