Posted in their Discord today in the Linux channel, which you only see if you've added yourself to a special role, was an announcement about Linux support ending. They've also already removed the SteamOS icon from the Steam store page.
Why? Well, it seems they've been having some issues with hackers recently. They're trying to sort it out but what's not helping is they've been unable to use IL2CPP, a Unity-developed scripting backend, on Mac or Linux:
Hi Penguin friends. We've supported Linux (and Mac) as long as we possibly could, despite the many hurdles (Mac and Linux getting updates/bug fixes/support last, lowest forms of support for dev packages, etc).
Now, we have 2 more major hurdles:
1) The recent hacks
2) I can't build on Linux a special way that keeps most code safe (IL2CPP) and efficient.
Here's the deal
* We have vulnerability fixes for the recent hacks, but not for Linux or Mac.
* We still cannot build in IL2CPP (that special way) on Mac or Linux.
Sadly, Linux support can only be as good as the game engine they build their game with. In this case, their issue is partly similar to what happened with Rust as no IL2CPP support for Linux being one of the reasons. Hopefully the Unity team will be working towards getting that working with Linux so we don't end up seeing more of this for online multiplayer games.
Much like Natural Selection 2, it's a small team struggling to keep a player-base going. It's not exactly a popular game, with it barely able to keep even 100 players online. Given that it needs a minimum of 8 people per game, that's not good. That said, I'm quite sad to see yet another game drop official Linux support.
There's a silver lining though for Linux gamers at least (seems MacOS support will never come back), as the Imperium42 staff member also said:
In the future, I may be able to bring back Linux support (but probably not Mac) since I can emulate Ubuntu significantly easier, but I still lack a build process and Unity doesn't behave well in Linux. I'd like to make a pipeline to upload via my server, but that won't be anytime soon, unfortunately
Hopefully this doesn't end up becoming a trend. Want to make sure it doesn't? Make yourself heard. If you buy a game that supports Linux and you enjoy it: tell friends, tell the developer, write a Steam user review for it and so on. Just don't be silent on it.
Hat tip to Expalphalog.