Dominant US-made desktop operating system Windows is falling in use. Libre and kind-of-global Linux is mostly growing, albeit slowly.

If you're not using "unknown" os, what are you even doing with your life.

"Unknown" is browsers blocking trackers from identifying them. So mostly Linux and some people running Firefox with blockers on other systems.

"Windows" is Windows users and also many Linux users who changed their bowser's user agent to circumvent sites that sabotage browsers on Linux for no reason.

"ChromeOS" is -again- Linux as it's based on open ChromiumOS which is derived from Gentoo. (And Google is planning to switch to Android soon... which is again running on top of Linux.)

AFAIK they're using the browser agent to identify the os. Maybe that obscures the os in some cases

This is taken from trackers, so blocking trackers renders your os as Unknown.

I really don't know. Neither the stats collector. That's the point.

Misconfigured systems/browsers or misconfigured web crawlers.

Based on the fact that it rises while Linux tails off, I'm guessing that it's something in Linux land breaking StatCounter's collection method. Maybe something like flatpacks and snaps not allowing the underlying OS to be detected properly.