Someone posted the following response from Apple support:
Just received a reply from Craig Michaels from Apple (I sent an email to Eddy Cue) regarding 24p output on the new Apple TV 4K:
Stijn,
Thank you taking the time to reach out about Apple TV. At this time, Apple TV 4K doesn’t support this output mode.
We appreciate your interest in Apple TV 4K and will take your feedback into consideration as we plan for future enhancements.
Best,
Craig
Hopefully this infomation is wrong because a lot of TV's can't do a reverse pulldown on a 60p signal!
Most noticable example: all high-end 2016/2017 Sony TV's with an X1-extreme processor.