YouTube has been supporting HDR, an extended dynamic range, for normally hosted videos for some time now. This option has now also been released for live streamed video transmissions. The formats HDR10 as well as HLG are available for selection.
In order to maintain the extended dynamic range during upload, compatible, HLS output-capable encoders must be used, and there are also some settings to be considered -- which ones are described in detail at 
this support page.
Of course, the camera used must also be able to record and output in HDR, and only those viewers whose TVs or monitors can display an extended dynamic range will see it. By the way, the preview in YouTubes Live Control Room shows only SDR even in HDR mode.
more infos at bei blog.youtube