That was a bigger thing in the past. Once upon a time engines were made with different metals that expanded/contracted at different rates. Then is was a good idea to let the engine get all up to the designed temp before putting any load on the engine. And carburetors were sensitive to temperature too. But modern engines with modern tolerances are all built out of one metal, mooting most of the reason behind letting the engine warm up completely. I wouldn't go racing on a cold engine, but running at 2000rpm driving slowly won't do more damage than idling at 1500rpm. Some even argue that doing so will get the engine up to temperature more quickly, reducing wear.