It's probably because they are existing apps. Apple wouldn't make technical exceptions for such a small market.
Can we see the text of the rejection?
Thanks for the response. Please find the rejection message below.
We also discovered one or more bugs in your app when reviewed on iPad and iPhone running iOS 9.3.5 on Wi-Fi connected to an IPv6 network. Specifically your app launches to a blank white screen with no content.
We've attached screenshot(s) for your reference.
Please run your app on a device while connected to an IPv6 network (all apps must support IPv6) to identify the issue(s), then revise and resubmit your app for review.
The vast majority of problems like this are caused by server-side configuration problems, possibly in the server itself but most commonly in the DNS. Have you read through the my Supporting IPv6-only Networks pinned post? What do you see when you work through the steps outlined in the post linked to by FAQ#6?
Share and Enjoy
Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware
let myEmail = "eskimo" + "1" + "@apple.com"
Thanks for the responses everyone
It turns out that it was unrelated to IPv6 changes. I needed to read the email a little better. It was the white screen on startup that was the real issue after all.
All is well now, and the app is live.