Plenty of high refresh gaming monitors out there that use HDMI 2.1 or DP1.4 with DSC.

DSC is just the "card up the sleeve" for DP1.4, you can push it all the way to 4K 240hz HDR. So there is little incentive to switch to DP2.1 for OEMs.

I can tell you mine does: a Corsair XENEON 27QHD240. Or the somewhat popular and badly named Acer Nitro XV275K P5biipruzx

I'd say it's usual for displays released in the 2-5 years old range. HDMI 2.1 got somewhat quick monitor support for consoles, but search for "first DP2.1 monitor" gets results from 2024

And that parenthetical is exactly why people care.

TVs.

lots of linux based boxes are used with TVs.