- The name of CEL(Common Expression Language) optional type has been changed from `optional` to `optional_type`. ([#124328](https://github.com/kubernetes/kubernetes/pull/124328), [@jiahuif](https://github.com/jiahuif)) [SIG API Machinery, Architecture, Auth, CLI, Cloud Provider, Network and Node]

    and is only suitable for experimental use.
 *  New: MockWebServer now supports client authentication (mutual TLS). Call `requestClientAuth()`
    to permit an optional client certificate or `requireClientAuth()` to require one.
 *  New: `RecordedRequest.getHandshake()` returns the HTTPS handshake of a request sent to
    `MockWebServer`.

* make using service account credentials from controllers optional ([#35970](https://github.com/kubernetes/kubernetes/pull/35970), [@deads2k](https://github.com/deads2k))

	if hasSessionPolicy {
		return isAllowedSP && (isOwnerDerived || combinedPolicy.IsAllowed(args))
	}

	// Sub policy not set, this is most common since subPolicy
	// is optional, use the inherited policies.
	return isOwnerDerived || combinedPolicy.IsAllowed(args)
}

func isAllowedBySessionPolicyForServiceAccount(args policy.Args) (hasSessionPolicy bool, isAllowed bool) {

* 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))

- EmptyDir memory backed volumes are sized as the minimum of pod allocatable memory on a host and an optional explicit user provided value. ([#100319](https://github.com/kubernetes/kubernetes/pull/100319), [@derekwaynecarr](https://github.com/derekwaynecarr)) [SIG Node]

### 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 new optional APIs in `ResouceSlice.Basic` and `ResourceClaim.Status.AllocatedDeviceStatus`. ([#130160](https://github.com/kubernetes/kubernetes/pull/130160), [@KobayashiD27](https://github.com/KobayashiD27)) [SIG API Machinery, Apps, Architecture, Node, Release,...

- Property `conditions` in `apiextensions.v1beta1.CustomResourceDefinitionStatus` and `apiextensions.v1.CustomResourceDefinitionStatus` is now optional instead of required. ([#64996](https://github.com/kubernetes/kubernetes/pull/64996), [@roycaihw](https://github.com/roycaihw))

- Use omitempty for optional Job Pod Failure Policy fields ([#126046](https://github.com/kubernetes/kubernetes/pull/126046), [@mimowo](https://github.com/mimowo)) [SIG Apps]

### Bug or Regression