### 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))

- Considerably reduced the CPU load in kube-apiserver while aggregating OpenAPI specifications from aggregated API servers. ([#71223](https://github.com/kubernetes/kubernetes/pull/71223), [@sttts](https://github.com/sttts))

<h3 id="Language_versions">Language versions</h3>

<p>
The <a href="/doc/go1compat">Go 1 compatibility guarantee</a> ensures that
programs written to the Go 1 specification will continue to compile and run
correctly, unchanged, over the lifetime of that specification.
More generally, as adjustments are made and features added to the language,
the compatibility guarantee ensures that a Go program that works with a

  optional ObjectReference involvedObject = 2;

  // This should be a short, machine understandable string that gives the reason
  // for the transition into the object's current status.
  // TODO: provide exact specification for format.
  // +optional
  optional string reason = 3;

  // A human-readable description of the status of this operation.
  // TODO: decide on maximum length.
  // +optional

In addition, users can now create multiple versions of CustomResourceDefinitions, but without any kind of automatic conversion, and CustomResourceDefinitions now allow specification of additional columns for `kubectl get` output via the `spec.additionalPrinterColumns` field.

### SIG Auth

1) Alpha for AWS ALB (Application Load Balancer) integration to Kubernetes Ingress resources.
2) Alpha for CSI specification 0.3 integration to AWS EBS (Elastic Block Store)

- Removed the networking `alpha` API `ClusterCIDR`. ([#121229](https://github.com/kubernetes/kubernetes/pull/121229),...

use to accurately and efficiently track set membership across `apply` operations. The format used for `ApplySet` is set out in  [KEP 3659](https://github.com/kubernetes/enhancements/issues/3659) as a low-level specification.  Other tools in the ecosystem can also build on this specification for improved interoperability.  To try the ApplySet-based pruning alpha, set `KUBECTL_APPLYSET=true` and use the flags `--prune --applyset=secret-name` with `kubectl apply`. ([#116205](https://github.com/kuber...

  <mime-type type="application/vnd.ms-wpl">
    <glob pattern="*.wpl"/>
  </mime-type>
  <mime-type type="application/vnd.ms-xpsdocument">
    <alias type="application/oxps"/>
    <_comment>Open XML Paper Specification</_comment>
    <glob pattern="*.xps"/>
    <glob pattern="*.oxps"/>
    <sub-class-of type="application/x-tika-ooxml"/>
  </mime-type>

  <mime-type type="application/vnd.msa-disk-image">

- In a future release, kubelet will no longer create the CSI NodePublishVolume target directory, in accordance with the CSI specification. CSI drivers may need to be updated accordingly to properly create and process the target path. ([#75535](https://github.com/kubernetes/kubernetes/issues/75535)) [SIG Storage]

#### kube-proxy: