Late reply! You may already know this, but I wanted to clarify to others that in plenty of American states, the age of consent is 16, while in others, it's 17 and 18.
However, when it comes to sexually explicit content in fiction stories, the universal minimum age is 18, as long as the content is written to titillate the reader, versus adult fiction where explicit sex may be included as part of the plot and not for titillation. All fiction texts are protected speech in America and therefore not illegal, but credit card companies still avoid that kind of content. As a result, virtually all sites that offer a payment system won't touch erotic content involving underage characters because payment systems are generally tied to credit card companies in some form or another.
Now, when it comes to illustrations, it's definitely illegal in America if the characters are under 18. So, the illustrations (not the text) were most likely the reason for the old version of the story being banned on Steam and not even available on itch. That is, assuming the old version included sexually explicit illustrations of underage characters.
I hope this helps!