for real people, depends on the person and the boundaries they themselves set tbh
I wouldn't read that one abo myl/ddd/yyq fanfic even if you paid me $100 because I find nsfw ff gross, but their teammate was literally reading it backstage; there's that gnz48 girl who joined so she could have better inspo for her fanfics and she gets along fine with her teammates; then there's that infamous tao finger slip where he posted that taohun ff onto his main
but since you don't get those boundaries set for most kpop idols, it's better to be safe than sorry