Natashi's GitHub pages, obviously, it's right there, just look up a little. This is where he will hopefully host his ph3sx documentations. Hopefully.
Touhou Danmakufu ph3sx is a modification of the engine Touhou Danmakufu ph3 (https://touhougc.web.fc2.com/products/th_dnh_ph3.html), geared towards optimizations and more advanced features.
Its main repository can be accessed here. https://github.com/Natashi/Touhou-Danmakufu-ph3sx-2
Please feel free to report any bugs and/or request any features in the "Issues" tab.
https://github.com/Natashi/Touhou-Danmakufu-ph3sx-2/releases