Thanks. It used to be possible to enable it months ago, but I'm not sure if it was actually displaying as HDR. I coulden't see much if not any difference at all. Compared to playstation 4 hdr, which shows big difference.
Since july or so, the hdr option has been greyed out, I think. It's been greyed out for me, for a while now, since I subscribed to pro.
I still have to try switching HDMI sockets, before I attempt to contact stadia about this issue.