Currently this branch (godot4 branch) is the latest branch, it's a work-in-progress port of the game to Godot 4.
In order to run the game, you'll need a specially customised version of the godot engine, this can be found here under the shinobu_gd4 branch. This modification of the engine includes steamworks support, ffmpeg and the new input glyphs system.
The current source code release is not open source, it's all rights reserved, but contributions are welcome with CLA. I still haven't decided what license I will use when it inevitably becomes open source.