My experience with HDR has been hit or miss as the updates come down in windows/nvidia. As of madVR 0.92.11, there is now a setting to let madVR decide how to handle HDR. And the newly released nVidia drivers 390.x series, now support the ability for madVR to seamlessly handle HDR playback in Media Player Classic with nVidia’s private API rather than using the Windows 10 API. So the HDR switch in windows is actually left off and madVR handles the rest. So now, windows is set to Auto for everything or FULL RGB and you can just play a file like you would anything else without any special setting changes.
Given the fact that Plex to my knowledge doesn’t use madVR and that madVR is the only true effective way for playing HDR content on the PC. I doubt Plex will be implementing this anytime soon given that the HDR functionality has finally stabilized with madVR. I know there are other ways for playing HDR content without madVR but those will require their own implementation at this point. And that’s what I see Plex doing, but no time soon.
Let alone transcoding / re-encoding on the fly to devices that may not support HDR or devices that do at lower bitrates. There’s more to Plex then just straight playing a file. Not so easy to implement imho.