In case someone is interested, it seems Anodyne's code is available on GitHub since April 2020:
https://github.com/analgesicproductions/Anodyne-1-Repo

Mind that while the full game can be compiled from the repository above, its license still requires the person who wants to compile the game to own the original game.

There's also a port for the Monogame engine, but just like with the original project, you still need to have purchased the game:
https://github.com/PixieCatSupreme/AnodyneSharp