What is the "kube-node-lease" namespace for?
here is the Kubernetes Enhancement Proposal of node-heartbeat.md for this feature. It makes node heartbeats significantly cheaper from both scalability and performance perspective.
This introduce a new Lease built-in API in the newly created API group coordination.k8s.io. which makes it easily reusable for other purposes 3rd party device monitoring plugins that's why it is namespaced.
This feature become stable in k8s 1.17