I have had both freesync and gsync workingwith no fuss for literal years. First with a 1070 and then with a 6800xt. I have never owned an HDR monitor so I can't personally speak to how well that works, but apparently it works just fine on the steam deck.

Even if it didn't, the number of people for whom HDR support is a make or break feature is likely not very high. Non developers use linux daily with the steam deck. I set up my mom with my xps 15 with Ubuntu on it when her laptop mobo died. While she wasn't a huge fan of having to learn a new UI, she was able to use it without issue for facebook/email/netflix/taxes/online banking for a few months until a good deal on a replacement came up and I got my laptop back.

I don't think if you are objective you can call linux so far behind windows. They are on par, each has their own annoyances and strengths, but for the average person they are just web browsing machines anyway.

HDR is coming. The OLED steam deck supports it, for example.

I'm on Fedora 40 KDE, both VRR and HDR are supported.