When you used a damage proc set, it would reset the timer for every damage proc set you are wearing. For example, if you were running viper, velidreth, and widowmaker in one build, and you attacked someone and proceed viper, it would make all the proc sets go to their respective cool down.
Proc sets would still be effective if you fit one in your build, but you won't be able to stack two or three sets into one build and have it insta killing people.
And reduce the damage off all of them across the board