A quick look at... the fumes coming out of a certain locomotive shows that a Linux version is indeed made available by GSC Game World. I remember this was also a key thing they highlighted while the game was being developed, and I remember being quite hyped about it back then.

Why is it not on GOG?

As a big fan of the series this really is the only thing that has prevented me from recommending getting this game here.

I'm guessing one of the top-rated reviews might have the answer, title:
"Multiplayer is not DRM-free"

Meaning it's one of those games that doesn't have a Linux version here because of no Galaxy support for Linux? :|

Really? Damn. That I'm painfully aware of...

Well, it's back to the first game in the series, I guess. It actually holds its own quite well even now and guess what - DRM-free LAN multiplayer included!

I'm guessing Cossacks 3 on Linux is pretty much abandoned then... sad times.

AFAIK Wine/Proton has broken water graphics (which possibly can be fixed to some degree by tinkering with game files), not working minimap and no multiplayer.

But native Linux is completely borked due to current libraries being not compatible with the game. I read on Steam that at least some time ago it was possible to play if you install Flatpak's version of Steam which still used older versions of libraries but it's of course only temporary solution.

While the broken water rendering is only mildly annoying, the lack of a minimap in a strategy game is really debilitating... you can't really play it in singleplayer either until that's addressed somehow. You can always soft link older .so-s from the OS that the game was developed on. I assume in this case it should be Ubuntu 18.04. Depending on what libraries are needed, it may or may not work.

GOG actually uses the same system for some of their older games, to ensure they run on the latest Ubuntu versions.