That was my first thought. That maybe it is Netflix saying we are going to protect our assets and you will need some legit way to view it.
As mentioned, this appears to be a DRM limitation, but imposed by Apple.
Netflix doesn't require it on Windows (obviously, since no non-Apple machines have T2), and is perfectly happy to stream 4K HDR HEVC titles on 2016/2017 Kaby Lake machines (and does it with low CPU usage).
Also note that Apple has been supporting hardware-accelerated 4K HDR HEVC (with low CPU usage) since 2017, and even highlighted that feature at WWDC. Take a look at Apple's table below, from WWDC 2017. "10-bit Hardware Decode" on "7th Generation Intel Core processor" means hardware 4K HDR HEVC acceleration on 2017 Macs with Kaby Lake CPUs.
And I'll post this again, since it demonstrates how well that hardware decoding works on all those 2017 Macs. Very low CPU usage (<10%) on the iMac 2017, and relatively low CPU usage (~25%) on the MacBook 2017, with 4K HDR bitrates much higher than what streaming video uses.
Here is High Sierra's QuickTime with my 1.2 GHz Core m3 MacBook (2017), playing back a very, very high bitrate 10-bit h.265 HEVC file. It's buttery smooth, with ~25% CPU usage. It was as expected, but nonetheless I'm pleased. :D The file is here: http://www.4ktv.de/testvideos/ Note that...
forums.macrumors.com
Seriously, if I had a 2018 iMac, looking forward to watching Netflix in 4k, and Apple tells me, sorry, you need a new Mac... Do you think I would spend the money on a new Mac to watch Netflix in 4k? Quite the opposite. I would be pissed off. And continue using my Mac for as long as possible. You sometimes find marketing people with that attitude, they usually get fired quickly because they don't actually sell things but drive people towards competitors.
I bought my 2017 MacBook and my 2017 iMac "knowing" they had the hardware to stream DRM'd 4K HDR HEVC. My prediction was that in 2018 Apple would release a new version of macOS that supported 4K DRM, on new 2018 Macs including a new 2018 iMac, and I also believed they would also support it on 2017 Kaby Lake models.
Well, I was wrong on several accounts:
1) Apple didn't even release a 2018 iMac. The next iMac wasn't until 2019.
2) Even the 2019 iMac didn't get supported.
3) The reason the 2019 iMac didn't get supported was because Apple chose to be more restrictive, requiring the T2 chip for this purpose, which mean that the iMac line didn't get supported until a new model was released in 2020.
I guess I bought at the right time after all. I bought in 2017 because my 2010 iMac was showing its age, but at the time I was afraid I had bought too early since Apple hadn't announced DRM'd 4K streaming yet. Well, while I did buy too early for this feature, it wasn't until three years later did finally Apple support it in iMacs, so I'm glad I bought when I did. Plus, it's been great using my 2010 iMac as a secondary monitor for my
dual iMac setup during all that time.