Maybe the answer is "when the time is right".

At least, steam is not working in IPv6 only client for now.("")

But we can see the UE4 changelog in 4.2 said it now support IPv6.

Why am I ask?
In my country China,most of families (about 99%)donot have public IPv4,three major ISPs provide people a private IPv4 ,they are using CGN(Carrier-grade NAT) for saving massive population requirement.At same time ,gov force ISP to provide every client a public IPv6(Maybe it's a good thing or bad thing),people donot noticed but it really happening.With a true public IPv6,community players can use their high-end PC build more game servers than expensive low-end cloud server.