AFAIK all of the studios only release the Atmos tracks with UHD movies - same as HDR, it's essentially a 4K-exclusive feature. It would not be smart for Apple to waste the money licensing Atmos on devices that don't play UHD in the first place, and they pretty explicitly said Apple TV 4K during the keynote. They didn't mention the 4th generation at all.
That’s not entirely correct. I can’t speak for ATMOS, but HDR encoding is available on 1080p movies, and is just as effective as a 4K movie. All of Netflix and Apple programs offered in DV or HDR will be displayed for 1080p versions as well as 4K. Amazon’s do not at this point, but that is likely to change if that is the industry norm.
Fair enough. The studios are doing their best to associate 4K with HDR and Atmos, though. They want people to adopt the new format.
Yes, HDR & Atmos* are being used as marketing points, however, the tech is not limited to 4K/UHD content. While there aren't any 1080p TVs that are capable of reproducing HDR, (that I know of), if you have an iPhone 8/8+ or X, or one of the most recent iPad Pro models, you can stream HDR content on those devices, (but, not at 4K resolution).
Transformers Age of Extinction 1080p blu-ray released in September of 2014 was the first blu-ray disc with Dolby Atmos, Gravity Diamond Luxe edition came out in 2015 w/Dolby Atmos, there are a few other titles that came out with Atmos audio before 4K/UHD discs started coming out also. Some studios release both the 4K/UHD & 1080p blu-ray discs with Dolby Atmos tracks, i.e. Blade Runner 2049, Wonder Woman, Star Trek Beyond, Deep Water Horizon, etc.
In 2015 HBO began releasing Game Of Thrones 1080p blu-rays with Dolby Atmos and went back and re-released previous seasons w/Dolby Atmos mixes on them.
So, it's not that it's not possible, it's just that they are using these for 4K/UHD marketing. (Also, what audio tracks are found on blu-ray discs depends on what licensing agreements/contracts the studios have with Dolby or DTS.)
So, for iTunes at least, whatever is available, if HDR10 or Dolby Vision, that will be available with the title, if Dolby Atmos is available, that will be available for that title, so, when you rent/buy, whatever your device is capable of streaming, the highest quality available, that is what you'll get out of iTunes, (except if you buy/rent SD). (Yes, Netflix and other apps can be updated to take advantage of Dolby Atmos.)
So, with all of that said, yes, I think Apple will add Dolby Atmos capability to the ATV 4th generation, however, we won't know for sure until it shows up in tvOS 12 beta.
*For UHD discs technically "Immersive audio" is the marketing point, that can be Dolby Atmos or DTS:X, however, not all discs will have immersive audio.
