It sounds like they were providing more than testing ipv6. I'm not apposed to putting up a test site if it only has to work with browsers. I would imagine something as simple as 2 DNS records A and AAAA references to 2 images. Just 2 png/gif stickers that say "IPv4 works!" and "IPv6 works!" and placeholders that show what is expected. Would more be required?
See https://www.kame.net/ for a basic browser test. If you see the dancing turtle, it means you're connected over IPv6.
It even works without Javascript.
Was sad to read this. Of course I understand his decision but it was a very useful site and helped IPv6 keep limping along its (barely) upward trajectory.
Hopefully someone will come along to carry the torch.