Here’s the meat of the reply I got back:
As a result of your app being removed from sale, TestFlight versions of your app are no longer available for both internal and external testers.
Existing testers can't install new builds, and existing builds will not be available after the expiration date.
If you need to test progress builds of your app, do so by distributing them to your registered devices.
To restore the TestFlight version of your app, you must resolve the current issues with the production version of your app.
I guess there’s not much they can do if your app was removed from sale because of a lack of updates. I think App Review differs between TestFlight and the App Store in its rigour, and so if you were booted from the store they really want to go in-depth before allowing a binary to go out to the public. That’s just my theory, with nothing to back it up, so if anyone knows anything more, I’m all ears.
Anyways, whatever. This isn’t a make-or-break app (or version) for the business, so I’ll do some smoke testing and just YOLO it onto the App Store.