I have gaming PC still running Windows 8.1 and have no plans to use "spyware" windows versions 10 and above.
I have PS5/Nintendo Switch for "modern" games.
I have Mac for real work.
I have couple of Linux boxes just for fun.

I used to play Xenonauts on Linux and Win 8.1 but if only 10 and 11 are supported that is a huge bummer.

I just have no way to play it :(

It is not a typo.

However, it's unclear if the requirements are written this way because the game simply won't work on below Win 10 or if the developer cannot guarantee that but it's technically possible to run it, Especially since the game is in ongoing development, which means things can always break in the future.

Previously in development people had reported that they could still run the game on below WIn 10, even when official requirements indicated Win 10 was necessary - see e.g. this thread from when X2 was in closed beta: https://www.gog.com/forum/xenonauts_2/win10_only

I don't know how things look right now - someone would have to try to run the game on an old OS as an experiment.

I have no desire to pay $30 just to do experiment :(

Too late for my case. the game actually run fine for the first 1~2 days after purchased. But since the game received the 1.21 update yesterday, the game will stop responding at 90% on the loading screen when entering any tactical battle and then crash to desktop. Restart the PC, load from previous saves or playing a new game and still same issue. The game is now not-playable for me at current stage. :(

Did you run it on Windows 8 or 10?

Window 10, anyway uninstall and reinstall seem to work now. On well, supposed is a norm for games in early access. :(

I don't have Win 8.1 on anything anymore :(

But I do have Win 7 and Win 10 and have played the latest build of X2 on both, and it works fine on both systems. Just my experience thus far!

Thank you for good news!
Just one wrinkle - Win 7 has directx12 backported. Win 8 does not. Still not sure if it will work on Win 8.1
Theoretically I could use win 7 but since I prefer to use SSDs I'd consider that as last resort. AFAIK win 7 was not designed to properly work with ssd and lacks trim support.