Maybe the answer is "when the time is right".
At least, steam is not working in IPv6 only client for now.("https://github.com/ValveSoftware/steam-for-linux/issues/3372")
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.