> I'm curious, how does Apple detect apps which don't support IPv6?
If their network is entirely IPv6 then they may not do anything special to test it. If an app fails with IPv4/IPv6 related errors then it doesn't support IPv6 properly.
And mind you, this means that your app needs to support IPv6 addressing. Your backend server doesn't need to support IPv6, since you can route to IPv4 addresses on IPv6-only networks.
If you're using the correct APIs with IPv4 addresses, you're fine.
If their network is entirely IPv6 then they may not do anything special to test it. If an app fails with IPv4/IPv6 related errors then it doesn't support IPv6 properly.