It's a matter of taste. Even though I'm developing a harem game, I play a lot of NTR games or read NTR manga.
But that's because I empathize with the villain, lol.
Anyway, yes, it's common to specify that kind of content because it might trigger someone's internal trauma.
Stupidly enough, it's like warning people that there are spiders in the game, haha.
It's the right thing to do, especially since both NTR and spiders in games are becoming more common.