yo cake I got something for yo idk if this helps but worth trying
https://github.com/teker821/ULTRAKILL/blob/master/ULTRAKILL.Cheats/ManageSaves.c..
Viewing post in ULTRAKILL PRELUDE WEB PORT comments
Um... that is the source code for the full game, not the prelude on itch.io. And besides, that does not do what you think it does. I don't truly know what it does, but I do know that it is not going to help me. So we will still be unable to save until someone looks at the code themselves. (I literally have the source code on Github. go look at it yourself if you know a way to fix the saving issue)
There is a green button on the page that should say "Code", click on it and a menu should appear. Click the bottom most button of that menu that says "Download ZIP" then you should see it start to download a zip. This contains the entire Unity project necessary to actually compile and distribute the game. You will need to know how to use Unity to some extent if you want to make drastic changes.