### 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))

    * If you are running E2E tests which require SSH keys and you utilize environment variables to override their location, you may need to modify the environment variable set. On all providers the environment variable override can now be either an absolute path to the key or a relative path (relative to ~/.ssh). Specifically the changes are:

* `hack/local-up-cluster.sh` now enables the Node authorizer by default. Authorization modes can be overridden with the `AUTHORIZATION_MODE` environment variable, and the `ENABLE_RBAC` environment variable is no longer used. ([#49812](https://github.com/kubernetes/kubernetes/pull/49812), [@liggitt](https://github.com/liggitt))

* CUSTOM_KUBE_DASHBOARD_BANNER environment variable will be used to populate notification banner in Kubernetes Dashboard add-on. ([#60639](https://github.com/kubernetes/kubernetes/pull/60639), [@konryd](https://github.com/konryd))

or `/etc/sysconfig/kubelet`, depending on the system...

- Fixes a 1.34 regression starting pods with environment variables with a value containing `$` followed by a multi-byte character ([#136491](https://github.com/kubernetes/kubernetes/pull/136491), [@AutuSnow](https://github.com/AutuSnow)) [SIG Architecture and Node]

enabled by default in all Kubernetes clients to fix persistently broken connections (https://github.com/kubernetes/client-go/issues/374). If needed, users can tune the feature via the HTTP2_READ_IDLE_TIMEOUT_SECONDS and HTTP2_PING_TIMEOUT_SECONDS environment variables. The feature is disabled if HTTP2_READ_IDLE_TIMEOUT_SECONDS is set to 0. ([#100376](https://github.com/kubernetes/kubernetes/pull/100376), [@liggitt](https://github.com/liggitt)) [SIG API Machinery, CLI, Cloud Provider, Cluster Lifecycle,...

information for OpenStack cloud provider can now be specified as environment variables. When we convert the OpenStack cloud provider to run in an external process, we can now use the kubernetes Secrets capability to inject the OS_* variables. This way we can specify the cloud configuration as a configmap, and specify secrets for the userid/password information. The configmap is mounted as a file, and the secrets are made available as environment variables. The external controller itself runs as a pod/daemonset....

to "systemd" unless the user is explicit about it. This requires existing machine setups to configure the container runtime to use the "systemd" driver. Documentation on this topic can be found here: https://kubernetes.io/docs/setup/production-environment/container-runtimes/. When upgrading existing clusters / nodes using "kubeadm upgrade" the old cgroupDriver value is preserved, but in 1.22 this change will also apply to "upgrade". For more information on migrating to the "systemd" driver or remaining...

- The certificate signer no longer accepts ca.key passwords via the `CFSSL_CA_PK_PASSWORD` environment variable. This capability was not prompted by user request, never advertised, and recommended against in the security audit. ([#84677](https://github.com/kubernetes/kubernetes/pull/84677), [@mikedanese](https://github.com/mikedanese))