CEC is actually implemented in Linux Kernel and you can use it (on supported hardware) with cec-client. So I’m not sure about being legally proprietary, but it’s part of the HDMI standard since 1.0 (thus, if you support it, you support CEC too) and it’s not at all a DRM.
Does this get HDMI CEC working?
Don’t think so. It’s currently focused on 4:4:4 colour at high bandwidth (4k@120hz), HDR, and VRR.
Isn’t CEC proprietary and effectively some form of DRM?
CEC is actually implemented in Linux Kernel and you can use it (on supported hardware) with cec-client. So I’m not sure about being legally proprietary, but it’s part of the HDMI standard since 1.0 (thus, if you support it, you support CEC too) and it’s not at all a DRM.