Apple is not forcing IPv6 only support, but they have to adapt to what's happening in the world. More and more mobile networks deploy NAT64 and if apps on Apple would not work on those networks then that could give Apple a bad reputation. Therefore they require that apps support such a network.

There are still apps in the app store that have been uploaded before the IPv6 requirement was in place so they may be broken. At the moment those apps are allowed to stay in the app store, but in the future they might be kicked out. Last time I asked Apple hadn't decided over that yet.

UDP should work through NAT64, but it will be implementation dependent how long ports will remain open etc. That will depend on the network, not on Apple.

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