What fan service thing? From your horikoshi post?
Anything Mineta does. Yaoyorozu panty shots, cleavage nonsense, the story bending over backwards to justify midnight’s everything, the way the story likes to call attention to the fact that Hagakure is naked fairly regularly just purely so that you’ll think about her being naked and not for any other reason.
Fanservice is a common term, especially in anime or video games, for sexualizing female characters for no reason other than to give the audience something sexy to look at or imagine doing. Usually, something becomes “fanservice” when it’s completely nonsensical and gross, like when the camera placement or costume design of a character is clearly mostly about their “assets”.
Horikoshi defends his fanservice pretty hardcore. He says that he’s a bit of a pervert himself so he likes Mineta. It’s the only thing about bnha that I hate and it isn’t going away anytime soon. This is a man who draws All Might the way he does because his editor complained that he didn’t like it and continues to do so out of spite. So yeah.