You can actually get HDMI CEC working on any Linux PC that has DisplayPort right now. That’s because DisplayPort 1.3 supports transporting CEC messages over the auxiliary channel. So a suitable DisplayPort-to-HDMI adapter will work for CEC, even if the native HDMI port doesn’t.
Unfortunately the “suitable” part is doing a lot of heavy lifting here. The cheap “passive” ones used for DisplayPort Dual-Mode will never work for CEC. You need an “active” adapter that uses a chip that supports CEC. These are known to work (pay attention to the model numbers!):
- CD0712: Active DP to HDMI Adapter 4K@60Hz | CableCreation
- CAC-1080: DisplayPort™ 1.4 to HDMI™ 2.0b HDR Active Adapter | Club3D
On Framework laptops, the HDMI Expansion Card also supports CEC, since the USB-C ports support DisplayPort Alt Mode.