Best to check the UK website but the US one says "This Onkyo AVR enables transmission of High Dynamic Range (HDR) formats, and provides enhanced picture quality by simultaneously enabling greater detail for both the dark and bright parts of an image. "
I expect they're HDMI 2.0a compatible, most are these days, but more importantly that receiver is only HDCP 2.2 compliant on three of its seven inputs. That may be enough for most people (Blu-Ray / DVD, satellite box and one other), but you should check your needs.
They've just unveiled the 6-FIVE-6 so look out for discounts.