Fair enough! I hadn't updated the app since the iPhones 6 came out or something. The 3rd of October, 2015, to be exact. And so I had 30 days to push a new version to the App Store.
I didn't get around to it, so the app was removed from sale. That's fine.
I'm now working on v1.3, which is essentially a simplified release of the original version (the v2 redesign is still underway!) and, having gotten to a good point for getting a dev build on TestFlight, I built and uploaded an archive, released it to internal testers, aaaaaannnd...
“The requested app is not available or doesn't exist.”
Apparently it's not just me. The app isn't near completion enough for getting through App Review, so I can't try releasing it to sale for a minute before re-removing it as suggested by some. I'll wait a day or two to see if this gets cleared out in the system, and maybe try reaching out to Developer Technical Support to see if there's a switch they can flip on their side to get internal builds working again.
In the meanwhile, I'll just set my development iPhone as an Xcode build target and test that way, and will follow up with updates.