A fair enough rule in my opinion as going around the battlefield dispatching unconscious opponents provides little role-play opportunity, doesn't advance the story and is a little bit ghoulish.
I don't know, the last time a player at my table grimaced reluctantly and said "[his character, TiefWiz] draws her knife and sets about her grim work" thus performing the coup de grâce on four sleeped goblins (I didn't ask for any rolls because I am sane), it really emphasized the delightful moral ambiguity at the heart of D&D. I like that it has so many problematic assumptions (so many of the games' underlying premises and so much of its history are racist that it's hard not to make fun of it while DMing certain modules, even with children present!) because it's an opportunity to confront and subvert those tropes... or not. Because it's your table and if you're an OG, you're an OG, respect.
Anyway I think leaving out these details sanitizes the violence, which is the opposite of how violence or warfare (D&D is a hack of a WAR-GAME) should ever be portrayed: I want my D&D to be more Dark Souls meets Full Metal Jacket than Star Wars meets Lord of the Rings.
(And D&D was always 70% Conan/Elric/Vance and 30% Tolkien at most, anyway.)
Also perhaps more important than any of that? I never suggested that they slit the throats of the sleeping subhumans. I actually didn't even ask them what they were going to do with the sleeped goblins. The party had an internal discussion and decided to cold-bloodedly murder them based on the assumptions of the game. Maybe I should have challenged them on that, but as the six of them had come to a consensus, that would feel like being a shitty DM. The morality, or lack thereof, of D&D, is interesting, if not important.
(Party of 6 was collectively strongly Chaotic Neutral. For the few CG characters, I didn't give them any grief. Goblins eat babies. It is known.)