How to use IPv6 for kubernetes service ip space?
I don't think IPv6 is fully supported. I don't think there is a strong motivation among the developers of the project to add IPv6 support, because the largest group of contributors is Google employees. Google Compute Engine (and thus Google Container Engine) doesn't support IPv6, so it wouldn't benefit Google directly to pay their employees to support IPv6. Best thing to do would probably be to pull in employees of companies that run their hosted product on AWS (as AWS has IPv6 support) such as RedHat, or try to contribute some of the work yourself.
From the linked PR, it looks like Brian Grant (Google) is, for whatever reason, somewhat interested and able to contribute IPv6 support. He'd probably be a good resource to query if you're interested in contributing this functionality to Kubernetes your self.
AWS already made IPv6 by default for almost all of their major services -- https://aws.amazon.com/blogs/aws/new-ipv6-support-for-ec2-instances-in-virtual-private-clouds/
Recently, the IPv6 support is accepted, one by another started too, in-fact, the POD implementation has done so far. k8 is moving towards Service and then issues.
Currently, the open blocker issues are still open with good use cases --https://github.com/kubernetes/kubernetes/issues/27398