The basic gameplay of Undertale in Battle is basically Touhou lite. The Design of Toriel and the Final boss (and, loosely, the Deltarune itself) were both inspired by the first Final Boss Touhou had, Sariel. Even the music is influenced from Touhou, the easiest one to notice is "Battle Against a True Hero". There's a lot more than that, but this is what Toby Said:
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As for the amateurs, that's more a "Milage may vary" thing. It's important to the doujin industy as it's basically a Free Use IP to do what you want with, so people just starting and selling their games at Comiketuse Touhou both as a means to sell their game and practice their own work. Of note, Former Mario Kart Double Dash developers are developing a Touhou racing game on the Switch.