I don't think they do. The 27" 5K on the latest models looks like it's their most advanced model, and it's 527cd/m2 and contrast ratio of 960:1, while the Ultra HD Alliance certifies HDR displays at a minimum of 540cd/m2 and 1,080,000:1 for LDC displays.

So they're super nice, I'm very happy with my 2017 model and it's the best display I've ever used, but not HDR.