I don't know. I have an LG TV and it does not support turning the display on/off with HDMI CEC. Everything else seems to work but it intentionally ignores those commands.
Option 1 (Hidden Menu Method)
* Press the Mute button repeatedly until the hidden menu appears; ensure Auto Power Sync is enabled.
* Go to General → Devices → TV Management and disable Quick Start+.
* Go to General → System → Additional Settings → Home Settings and turn off both options.
Option 2 (Settings Menu Method, webOS)
* Press Settings on the remote and open All Settings.
* Navigate to General → Devices.
* Turn SIMPLINK (HDMI-CEC) ON. (webOS 6.0+, enabling SIMPLINK automatically enables external device control).
https://learn.microsoft.com/en-us/windows/win32/api/highleve...
Displayport has DDC/CI, which allows you to adjust things like brightness, volume, etc. remotely. This has existed since the DVI era (!) which means Displayport has a huge headstart. But they never formalized and enforced the DDC/CI spec, which means every monitor has extremely weird quirks. Some will allow you to send and read data. Some will only allow you to send data and crash when you try to read. Some will update only once every few seconds.
Although in this specific case, one wonders why Valve didn't just use two Displayport 1.4 ports and and stuck an onboard HDMI converter in front of one of them, sourced from a company that would be amenable to having Valve work on the firmware of said converter. Make the entire firmware of the converter open source except for the binary blob that handles the Displayport 1.4 -> HDMI 2.1 bits.
Hopefully Valve does this but sells it as a external, high quality converter. It would be a nice little plus even for non-Steam Machine owners, same way like Apple's USB-C to 3.5mm convertor is the highest quality mini DAC on the market for the low price of €10.