* In-place node upgrades to this release from versions 1.7.14, 1.8.9, and 1.9.4 are not supported if using subpath volumes with PVCs.  Such pods should be drained from the node first.

* The minimum supported version of Docker is now 1.11; if you are using Docker 1.10 or below, be sure to upgrade Docker before upgrading Kubernetes. ([#57845](https://github.com/kubernetes/kubernetes/pull/57845), [@yujuhong](https://github.com/yujuhong))

  
  vSphere releases less than 67u3 are deprecated as of v1.21. Please consider upgrading vSphere to 67u3 or above. vSphere CSI Driver requires minimum vSphere 67u3.
  
  VM Hardware version less than 15 is deprecated as of v1.21. Please consider upgrading the Node VM Hardware version to 15 or above. vSphere CSI Driver recommends Node VM's Hardware version set to at least vmx-15.

- The resource version option, when passed to a list call, is now consistently interpreted as the minimum allowed resource version.  Previously when listing resources that had the watch cache disabled clients could retrieve a snapshot at that exact resource version.  If the client requests a resource version newer than the current state, a TimeoutError...

*   The Go version was upgraded from go1.8.3 to **go1.9.2** ([#51375](https://github.com/kubernetes/kubernetes/pull/51375),[ @cblecker](https://github.com/cblecker))
    *   The minimum supported go version bumps to 1.9.1. ([#55301](https://github.com/kubernetes/kubernetes/pull/55301),[ @xiangpengzhao](https://github.com/xiangpengzhao))

The [dynamic maximum volume count](https://github.com/kubernetes/enhancements/issues/554) feature was also moved to beta. This enables a volume plugin to specify the maximum number of a given volume type per node as a function of the node characteristics (for example, a larger limit for larger nodes, a smaller limit for smaller nodes).

* Updated kubeadm's minimum supported kubernetes in v1.11.x to 1.10 ([#63920](https://github.com/kubernetes/kubernetes/pull/63920), [@dixudx](https://github.com/dixudx))

- Kubelet allows pods to use the `net.ipv4.ip_local_reserved_ports` sysctl by default and the minimal kernel version is 3.16; Pod Security admission allows this sysctl in v1.27+ versions of the baseline and restricted policies. ([#115374](https://github.com/kubernetes/kubernetes/pull/115374), [@pacoxu](https://github.com/pacoxu)) [SIG Auth, Network...

- The default Go version was updated to 1.11.2. ([#70665](https://github.com/kubernetes/kubernetes/pull/70665))
- The minimum supported Go version was updated to 1.11.2 ([#69386](https://github.com/kubernetes/kubernetes/pull/69386))
- CNI is unchanged at v0.6.0 since Kubernetes 1.10 ([#51250](https://github.com/kubernetes/kubernetes/pull/51250))

### Bug or Regression

- Fixed a bug where a high churn of events was causing master instability by reducing the maximum number of objects (events) attached to a single etcd lease. ([#100452](https://github.com/kubernetes/kubernetes/pull/100452), [@mborsz](https://github.com/mborsz)) [SIG API Machinery and Instrumentation]

* [Federation][kubefed]: Use StorageClassName for etcd pvc ([#46323](https://github.com/kubernetes/kubernetes/pull/46323), [@marun](https://github.com/marun))
* Restrict active deadline seconds max allowed value to be maximum uint32 ([#46640](https://github.com/kubernetes/kubernetes/pull/46640), [@derekwaynecarr](https://github.com/derekwaynecarr))