How can I sync the new "people" (face recognition) metadata across devices in Photos (iOS 10 and MacOS Sierra)?
Before High Sierra release (and the corresponding iOS annual release) this metadata did not sync. Now that data does sync.
Macworld speculates on the reason behind it:

It’s not clear yet whether [facial recognition data will ever sync] because Apple’s push at its recent [2016] Worldwide Developers Conference was for local analysis of private information that’s never uploaded to the cloud.

I haven't watched it myself, but apparently Phil Schiller and Craig Federighi stated on John Gruber's The Talk Show that it will not happen, that the features are device-specific and will not be synced via iCloud.
Edit: The help file for Photos in macOS Sierra was updated with this:

Note: People identified in the People album are not synced across devices.