It was made in Smile Game Builder. The demo is on booth, but I think you're suppose to get the full game through Pixiv Fanbox which is like Patreon I guess, you have to pledge so much money to see content.
I'm not confident he'll follow though. Lots of artists experiment with RPG development, but the majority will cancel their projects and go back to comics and CG Illustration packs. I guess a continuing narrative that extends beyond a picture and a bed room is harder than people realize.
Save yourself heartache and don't follow things that are "in development".