I think It might've been last year but anti-cheat compatibility was huge. Still up to the studio to enable it but some games have.
Progress towards wayland is going steady which doesn't mean much to those that don't know much about Linux. But what that means is more modern features like VRR and HDR. Not completely here yet as far as I know but that wouldn't have happened on X11
Note exactly a huge milestone but you can't discredit the steady development of pipewire. Audio was annoying as fuck before it.
Honestly, I think we're 3 years out from Windows being replacable for a gaming platform.
Anti-cheat is a big one (sure, there's "support", but if none of the games people play are supported, is that support?), but VRR and HDR are also huge.
That trifecta is the only reason I'm still sitting in Windows, and I find myself hopeful we land there sooner rather than later so I can dump Windows and never have to think about whatever dumb crap Microsoft is going to do next.
VRR works really well already - some Nvidia users might lose extra functionality like Reflex Ultra that, when paired with VRR, can smartly adjust the frame rate cap. But VRR itself works.
HDR is a difficult beast though... It's hard even on Windows, and very problematic on Linux (though with Gamescope, KDE Plasma and Wayland you can kinda use it already).
I've been using HDR on Linux since February or March and it works pretty well. MPV works great (with vk_hdr_layer), and games work if you run them in Gamescope, which has its own complications but overall it's pretty good.