There are competitions for all sorts of retro computer and game console platforms which are "purer" in the sense that there's little to no operating system at all, so the demo has to be programmed against the bare metal. That's not feasible for modern PCs though, you need the OS infrastructure and drivers to abstract over the variety of hardware.

Or if you're web-inclined you could use the browser as your OS:

https://0b5vr.com/domain/domain.html / https://www.youtube.com/watch?v=D2COWeeEqTs

https://fms-cat.github.io/condition/pages / https://www.youtube.com/watch?v=co42e8ErbaY