itch.io

I know what proton is, but you already have proper linux version here that you can simply upload there as well, which is why im surprised.

Its strange that steam version doesn't have linux support while the one here on itch has it. And mac too, i guess.

As was mentioned below already, there is problem with Linux version. I was sort of able to launch it my laptop (with Alt Linux 10), but it doesnt work at all on my main PC (with Ubuntu Linux 20.04).

I dont know how specifically you do your build, author, but here is main tips on how to do it right when it comes to linux games(or any other linux apps, actually):
1) First,  including any rare libraries that may not be present in system is almost always a good idea
2) Unless you want some specific version of the lib(usually newer one), than its better to build your game on the oldest supported linux distro to link with the old versions of libs. That's because compatibility usually work only one way.  Old versions work with new systems, but not opposite.
3) The other option is to link everything statically and carry all the game libs with you, that's how its usually done on windows, btw.

Or just use the latest version of engine from its official site. That works, too. Just tested it on my ubuntu, where original version from game pack did not work.

Well, unless deleted specifically, renpy novels usually include everything needed for launch on linux by default in my experience. Linux section in "lib" folder together with .sh file in game root is exactly that.
Not the first time i see authors claiming their games to be win-only, with game being, in fact, cross-platform. :)

Why does game claim to be windows-only (in download section) when its clearly RenPy-based, and has linux-specific files included? Just tried to launch it and it works fine on my ubuntu linux.