2019, at earliest. Why?
#1. Atmos is new, with a relatively small user base. Apple takes it's time with new standards,
unless they feel it will help move a lot of new hardware.
#2. By removing audio passthrough on the ATV, I worry Apple may have painted themselves into a corner. Now they'll have to pour R&D into transcoding the Atmos audio into PCM, versus simply passing the Atmos stream onto the AVR.
Just a guess, though...