"On June 15, Apple rejected an update to a previously-approved iOS app called Hey. The app is for a new email service created by a company called Basecamp.
The rationale for said rejection was that Hey offered a subscription via-website without also offering a parallel subscription via Apple's in-app purchase (IAP). Apple takes a 30% cut of IAP revenues during an app's first year, and 15% thereafter."
This site has dozens of examples of apps that you can download from the iOS app store that don't work without a subscription you purchase elsewhere. This makes the rejection of the new email app Hey seem like Apple is singling out a potential direct competitor rather than evenly enforcing an existing policy.
youdownloadtheappanditdoesntwork.com
More background: Apple doubles down on controversial decision to reject email app Hey.
Update (6/23): We're all good here? Apple, HEY, and the Path Forward.