Awful but not really new, is it? Just a new word for it.
Actually, I remember men/boys doing this to women/girls even when I was young. Saying, "I'll call you" and then never calling. But in that case, I understand. It's hard to say to someone's face, "I don't want to see you again."
But when someone does this to family or friends, I don't get it. If one is going to go nc, why not at least let the person know? Cowardly and cruel not to, imo.
If one has already told the person what's bothering them and it still doesn't stop, I can see not explaining again. But how about, "Iv had it Mom/ds/old friend/fill-in-the-blank! I won't be talking to you anymore!" It might hurt, but at least they would know what was going on. They might argue, but one could just say, "Sorry, I'm done" and end the conversation.
I'm not a fan of cos, but if one's going to do it, why not be honest about it?