Man, I really hate hyper-boobs like this, though most of the time it's easy to ignore, but christ it feels like 60% of Karryn's body weight is in her boobs and looks hard to ignore.
Loli/shota- Summer Memories, Melty's Quest- shota sold as "manlet" or halflings
School anything- President Yukino
Steam has taken games that have those elements before, the problem is on which valve employee are you gonna run when submitting your game because apparently they have a serious case of triple standards or some of them are very prudish.
These are games that slip through and if you notice, a lot of them have the content stripped out (Summer Memories) or really deep into the game with hard to verify media (it's not plastered all over the press content, none of the main cast looks loli/shota or whatever, it's not easy to find images of content like bestiality/school hentai and attach it to the game, etc). That said, there are also a lot of games that take place in a high school setting like Kozue's Strange Journey, but don't push it very hard.
Steam doesn't have triple standards, they have people very loosely verifying something they want nothing to do with. You expect them to start delving into porn sites or playing porn games to see if there's actually lolicon, then call over their boss and go, yeah, that's a 6-year-old getting railed by a horse, sir!
It's so wishy-washy because developers don't call attention to this content and it's not always easy to tell without referring to exterior sites, many of which Valve is unliekly to want to deal with, like Dlsite. So they make best case judgements, and the easier it is to see a problematic element, or the more the game is known for it, the more likely it is to get banned.
Stuff like Melty's Quest has its problem content so deep that the only people who will ever see it are fans, so no one is going to report it. So hey, I get it, I'm salty that Steam is basically firing blind when it comes to banning porn games and VNs, but they aren't doing it out of spite and I'd rather take some porn games on Steam than none at all.