From what iv seen anything with flat chested females is able to be labeled as young by steam and not allowed. given the design of these characters i would say its ability to be on steam is questionable at best.
That is true in most cases.
But there exist games that has flat chested young characters on Steam, but that's because the creators of those has cut off the sexual content that originaly exist in those. Like ,Nekopara is one of them, its just cut off all the sexual content so that it only has the wholesome and cute stuff in it, while the full content of it are in a DLC, so it only depends on how you make a game look to the masses.
But honestly, people these days looks at games and acts like a Karren when they see small characters, instead of just seeing it as a fantasy character.