Sansa was never taught this.
This lesson was likely never taught to Sansa or Arya to begin with. In the original scene, the lesson was for the benefit of the male children present (Robb, Jon, Bran). Given the attitudes towards what was proper for women to learn, he would not have bothered teaching this bit of wisdom to either Sansa or Arya (though Arya might have heard about it secondhand).
Sansa would have no reason to want to strike the blow herself. In addition, she has been learning, the hard way, the art of manipulation. It would fit with both this change in her personality, as well as her upbringing as a lady to let another hand do the dirty work.
She may have become aware of this later, but not likely - it's not the sort of thing that would come up in conversation before she was spirited away by Littlefinger, and it certainly was not among the behaviors he was trying to teach her. If she has not had occasion to pass this sentence on someone, there is zero reason for anyone to mention it. Certainly no one had the temerity to bring up at that moment.
Think of where she began, and where she is now. That sort of thinking belongs to the bright, honorable world she was expecting long ago when Robert and friends first came to town, a world view that has long since been shattered. Even if she IS aware of the tradition, it belongs to a world that must appear to her as naive and fanciful, where people do the right and honorable thing. she's seen what happens to people that think that way.
Whether she knows of it or not, she has no reason to do the deed herself.