This version of the game on itch.io does indeed require a mod for 18+ content. (A disclaimer of it not being the 18+ version along with a way to mod it would be nice devs.) I confirmed it earlier today and the only way to apply the mod is to find the files online in a roundabout manner since the developers did not bother to create a separate link for itch.io.
After hours of playtime, I saved the game the day before an event triggered (Month 4 Day 21). I then applied the mod, booted up the game again to replay the scene, and found nothing had changed. It might be because of how the scene played out that caused the mod to not work (it skipped a day without saving - Amy's route) or that the mod might require an entirely new save. So be aware that you might need to restart your game.