I got an email from them:
Your app, Temple of the Abyssal Winds, app Apple ID: 816495128, does not follow one or more of the App Store Review Guidelines.
Turns out there isn’t a problem with my app, it just hasn’t been compiled and submitted “recently”. So now I have to think about whether it is worth the trouble of digging out the old code , getting it to work with the new compiler (and dependencies), and go through submission again. It would probably take me a week to get through all that.
Is this really a problem? I guess not; it’s probably a good thing for users to ensure that apps are maintained and kept up to date.