Digging through macOS 10.13.4, I noticed that it included privacy information for a Podcasts app, which is currently only available on iOS (on macOS, podcasts are bundled in iTunes).
This privacy information indicates that a user’s podcast listening is linked to a random device identifier which is synced through iCloud in a way that cannot be linked to other Apple services. Apple collects data about users’ listening habits to share with podcast publishers.
The most interesting part is that it specifically mentions “Your use of the Podcast app is linked to your Apple ID”. We can’t be sure whether it’s just a typo and/or whether they left this privacy information on macOS by accident, but this could mean we’re going to see a Podcast (or Podcasts) app for macOS in the future.