sorry if i didn't make myself clear, im assuming you refer to 18+ as nudity and stuff like that but that's not the only factor that can make a novel 18+, you even list a lot of other type of things that qualify as mature content 😓
depicting mature matter does not automatically make a game 18+ Psychological horror, mental illness, violence, and disturbing themes can exist in media intended for audiences under 18. Content warnings are there to show players about upsetting material, not to automatically classify everything as adults only There is a difference between a game containing mature themes and a game requiring an 18+ rating