They don't, I can 100% guarantee you this. There are plenty of games on Steam that has loli/shota characters where they only change the age, loli/shota characters with lewds added in through patch, loli/shota characters that stay completely the same but they have no lewds. And they are 100% aware that the same game is being sold elsewhere with all the content they don't allow.
Main point of evidence: Kagura Games, do the research yourself.
All I'm saying (others opinions are their own) is that there's no need to change the character for the story. If they wish to add lewds for her, just give her a special chapter that takes place when she is of age. Simple as that.
Edit: Sorry, bit of clarification. Steam does not care about patches outside of their site, but yes any works with H they want all characters to be 18+ (some still get by visually but that's more high end company than indie works). So yeah, it's better to just change her to get by Steam and if RNG so chooses add original version back in through patch for story elements. For H, nah still a hard pass for me.