For Linux, when you exclude Android and ChromeOS. So for desktop GNU/Linux. And even then I'm not sure now, because Steam Deck and Manjaro use KDE, and Linux Mint uses not-gnomish-fork-of-gnome.
Its the default on those distros because those distros invest on GNOME and GTK and GTK is SW.
QT has an history of being connected with non-free SW licenses, therefore commercial (or company sponsored) for those distros to rely and invest on such project is risky.
I'd bet most CentOS and RHEL installations don't use any desktop environment at all.
However, why argue if we can check the actual stats? [1] GNOME seems to be well ahead for now for desktop GNU/Linux. But the dip in the February, when the Steam Deck was released, is interesting.
> However, why argue if we can check the actual stats?
Because nobody has actual stats. Your link is from one attempt to find some stats, but unless every major distro activates it by default (which they will probably never do on account of privacy issues), it's a tiny and biased sample.