Is kubebuilder/controller-runtime still maintained
Yes, the kubernetes-sigs/controller-runtime is still actively being developed on and is maintained.
I am wondering when could kubebuilder/controller-runtime update to k8s 1.12.x?
The best way to ask the maintainers to make this happen would be to ask on the #kubebuilder channel on the Kubernetes Slack, or create an issue on the repo.