Okay but Sans not giving a shit is 2023 fanon in the exact same way that Sans having an anime sobbing scene over piles of Papyrus's dust is 2016 fanon. Sans very much does give a shit. He goes blank-eyesockets over it. He calls you a dirty brother killer. He's just also terminally depressed, and not a particularly confrontational person, and he knows from his research into the Anomaly that he can't really hurt you in any way that matters. The fandom's way overcorrected for 2016's mistakes; Sans gives a whole lot of a shit, it's just that he doesn't wear his heart on his sleeve, and he struggles with converting that into anything particularly actionable. And who among us hasn't had such a difficulty?