### ConfigMap
* Volumes and environment variables populated from ConfigMap and Secret objects can now tolerate the named source object or specific keys being missing, by adding `optional: true` to the volume or environment variable source specifications. ([#39981](https://github.com/kubernetes/kubernetes/pull/39981), [@fraenkel](https://github.com/fraenkel))

- Added optional `--detect-local-mode` flag to kube-proxy. Valid values are "ClusterCIDR" (default matching previous behavior) and "NodeCIDR" ([#87748](https://github.com/kubernetes/kubernetes/pull/87748), [@satyasm](https://github.com/satyasm)) [SIG Cluster Lifecycle,...

### Cloud Provider

- The GCE-only flag `cloud-provider-gce-lb-src-cidrs` is now optional for external cloud providers. ([#76627](https://github.com/kubernetes/kubernetes/pull/76627), [@timoreimann](https://github.com/timoreimann))

- Adds support for `InterfaceNamePrefix` and `BridgeInterface` as arguments to `--detect-local-mode` option and also introduces a new optional `--pod-interface-name-prefix` and `--pod-bridge-interface` flags to kube-proxy. ([#95400](https://github.com/kubernetes/kubernetes/pull/95400), [@tssurya](https://github.com/tssurya))

* The `serverAddressByClientCIDRs` property in `metav1.APIGroup`(discovery API) is now optional instead of required. ([#61963](https://github.com/kubernetes/kubernetes/pull/61963), [@roycaihw](https://github.com/roycaihw))

- Mark Job onPodConditions as optional in pod failure policy ([#120209](https://github.com/kubernetes/kubernetes/pull/120209), [@mimowo](https://github.com/mimowo)) [SIG API Machinery and Apps]

### Feature

* OpenStack cloud provider supports Cinder v3 API. ([#52910](https://github.com/kubernetes/kubernetes/pull/52910), [@FengyunPan](https://github.com/FengyunPan))
* kube-up: Add optional addon CoreDNS.  ([#55728](https://github.com/kubernetes/kubernetes/pull/55728), [@rajansandeep](https://github.com/rajansandeep))
    * Install CoreDNS instead of kube-dns by setting CLUSTER_DNS_CORE_DNS value to 'true'.

### Network

* Fixed kube-proxy to work correctly with iptables 1.6.2 and later. ([#60978](https://github.com/kubernetes/kubernetes/pull/60978), [@danwinship](https://github.com/danwinship))

* Makes the kube-dns addon optional so that users can deploy their own DNS solution. ([#57113](https://github.com/kubernetes/kubernetes/pull/57113), [@wwwtyro](https://github.com/wwwtyro))

* Mark the LBaaS v1 of OpenStack cloud provider deprecated. ([#52821](https://github.com/kubernetes/kubernetes/pull/52821), [@FengyunPan](https://github.com/FengyunPan))
* Mark image as deliberately optional in v1 Container struct.  Many objects in the Kubernetes API inherit the container struct and only Pods require the field to be set. ([#48406](https://github.com/kubernetes/kubernetes/pull/48406), [@gyliu513](https://github.com/gyliu513))

- [`KT-58665`](https://youtrack.jetbrains.com/issue/KT-58665) K2: Optional.of incorrectly accepts nullable String
- [`KT-58938`](https://youtrack.jetbrains.com/issue/KT-58938) K2. Abstract class can be invoked using member reference `::` operator