Extensibility. SIG API Machinery added a new class of admission control webhooks (mutating), and brought the admission control webhooks to beta.

### Apps

The core workloads API, which is composed of the DaemonSet, Deployment, ReplicaSet, and StatefulSet kinds, has been promoted to GA stability in the apps/v1 group version. As such, the apps/v1beta2 group version is deprecated, and all new code should use the kinds in the apps/v1 group version.

### Auth

- The `controller.kubernetes.io/pod-deletion-cost` annotation can be set to offer a hint on the cost of deleting a `Pod` compared to other pods belonging to the same ReplicaSet. Pods with lower deletion cost are deleted first. This is an alpha feature. ([#99163](https://github.com/kubernetes/kubernetes/pull/99163), [@ahg-g](https://github.com/ahg-g))

### Other (Cleanup or Flake)

* fix typo  ([#59619](https://github.com/kubernetes/kubernetes/pull/59619), [@jianliao82](https://github.com/jianliao82))

* DaemonSet, Deployment, ReplicaSet, and StatefulSet objects are now persisted in etcd in apps/v1 format ([#58854](https://github.com/kubernetes/kubernetes/pull/58854), [@liggitt](https://github.com/liggitt))

    * replicas, readyReplicas, currentReplicas, and updatedReplicas. The semantics of replicas is now consistent with DaemonSet and ReplicaSet, and readyReplicas has the semantics that replicas did prior to this release.

- When scaling down a ReplicaSet, delete doubled up replicas first, where a "doubled up replica" is defined as one that is on the same node as an active replica belonging to a related ReplicaSet.  ReplicaSets are considered "related" if they have a common controller (typically a Deployment). ([#80004](https://github.com/kubernetes/kubernetes/pull/80004), [...

### SIG Storage

- The AllowedTopologies field inside StorageClass is now validated against set and map semantics. Specifically, there cannot be duplicate TopologySelectorTerms, MatchLabelExpressions keys, or TopologySelectorLabelRequirement Values. ([#66843](https://github.com/kubernetes/kubernetes/pull/66843), [@verult](https://github.com/verult))

In particular, the following storage features are moving to GA in Kubernetes 1.18:
- Raw Block Support: Allow volumes to be surfaced as block devices inside containers instead of just mounted filesystems.
- Volume Cloning: Duplicate a PersistentVolumeClaim and underlying storage volume using the Kubernetes API via CSI.
- CSIDriver Kubernetes API Object: Simplifies CSI driver discovery and allows CSI Drivers to customize Kubernetes behavior.

- The metric `apiserver_flowcontrol_request_concurrency_limit` has been deprecated and will be removed in a future release.  It is a duplicate of `apiserver_flowcontrol_nominal_limit_seats` (introduced in release 1.26) but has an outdated name and had an outdated HELP string. ([#118959](https://github.com/kubernetes/kubernetes/pull/118959), [@MikeSpreitzer](https://github.com/MikeSpreitzer))...

* `make test-cmd` now works on OSX. ([#61393](https://github.com/kubernetes/kubernetes/pull/61393), [@totherme](https://github.com/totherme))
* Conformance: ReplicaSet must be supported in the `apps/v1` version. ([#61367](https://github.com/kubernetes/kubernetes/pull/61367), [@enisoc](https://github.com/enisoc))