Apparently "almost no PC GPU has hardware support for CEC" according to Arch. Wonder if that is outdated and modern GPUs do?

https://wiki.archlinux.org/title/HDMI-CEC

This is still pretty much the case. There are ways to do it (i.e. adapters that sit between your HDMI cable and GPU) but it’s wonky.

The Steam Machine will support CEC, hopefully other PC vendors will take note and adopt it.