The story so far:
I normally play on Windows, mostly because I like to have the latest video drivers on my gaming system. But recently I decided to try installing Zero-K on a Linux Mint partition. Got the Zero-K semi-portable file from itch.io. Noticed that there's a complaint in the comments that the libcurl3 library is no longer available. So what if libcurl4 superceded it? Everything still works. Zero-K_linux64.sh wouldn't run at first because my TERM variable was set for 256 colors... Fine, set that to xterm and run. No update screen ("Zero-K is updating, please wait..."), but the game runs. OK, how do I get my update screen? Well, just running the "old" script does that for me. Great, everything runs. On the wrong monitor. Fine, I'll keep dragging the window where I want it (a nightmare, but not limited to ZK). But the game version remains at 1.8.9 no matter what.
Question of the day:
How do I update my game?
Other questions:
I didn't try to create a new account or enter my old one, it's just a local game for now. Could that cause it to not update properly? If so, that feels like a bug.
Is the itch.io version no longer maintained?
Conclusion:
I will probably remove this installation. Sadly, it runs worse on Linux than it does on Windows on the same machine, with strange micro-freezes despite fewer backgrounds processes (nothing in the log to explain them afact) and windows that don't stay on the monitor that I put them on. Nevertheless, I'd still like to know how to update it in case I need to help someone else who might have the same issue down the road.