We added +mat_hdr_level 0 in cxgames 7.x because the game tried to activate it, but it was broken(green quad poping up on the screen every now and then). Unfortunately during the upgrade process from 7 to 8 we cannot really tell options set by us from options set by the user, and we decided that not touching them was safer than trying to remove the old cut-down-the-features options. Thus +mat_hdr_level 0 was left in place.
Besides this, HL2 doesn't always change the dxlevel if it finds that the GPU is more capable suddenly. It changes it if the GPU changes, but not if the card stays the same, but the features change.
I tested HDR on my machines, and it worked in HL2:EP1 (same engine as HL2:Lost Coast), HL2:EP2 and TF2. I didn't test other HL2 based games, but I expect HDR to work there as well. Note that base HL2, CS:S and DoD:S don't support HDR even on Windows.