The `apiextensions.k8s.io/v1beta1` version of `CustomResourceDefinition` is deprecated and will no longer be served in v1.19. ([#79604](https://github.com/kubernetes/kubernetes/pull/79604), [@liggitt](https://github.com/liggitt))

### API Change

- Added `ObservedGeneration` to CustomResourceDefinition conditions. ([#134984](https://github.com/kubernetes/kubernetes/pull/134984), [@michaelasp](https://github.com/michaelasp))

- Kube-apiserver: fix a possible panic validating a custom resource whose CustomResourceDefinition indicates a status subresource exists, but which does not define a `status` property in the `openAPIV3Schema` ([#135363](https://github.com/kubernetes/kubernetes/pull/135363), [@fusida](https://github.com/fusida)) [SIG API Machinery]

- Alpha support for field selectors on custom resources has been added. With the `CustomResourceFieldSelectors` feature gate enabled, the CustomResourceDefinition API now allows specifying `selectableFields`. Listing a field there enables filtering custom resources for that CustomResourceDefinition in list or watch requests. ([#122717](https://github.com/kubernetes/kubernetes/pull/122717), [@jpbetz](https://github.com/jpbetz))

- CustomResourceDefinition objects created with non-empty `caBundle` fields which are invalid or do not contain any certificates will not appear in discovery or serve endpoints until a valid `caBundle` is provided. Updates to CustomResourceDefinition are no longer allowed to transition a valid `caBundle` field to an invalid `caBundle` field, because this breaks serving of the...

* The deprecated `ThirdPartyResource` (TPR) API was removed.
  To avoid losing your TPR data, [migrate to CustomResourceDefinition](https://kubernetes.io/docs/tasks/access-kubernetes-api/migrate-third-party-resource/).

* The following deprecated flags were removed from `kube-controller-manager`:

- Added CEL runtime cost calculation into ValidatingAdmissionPolicy, matching the evaluation cost
  restrictions that already apply to CustomResourceDefinition.
  If rule evaluation uses more compute than the limit, the API server aborts the evaluation and the
  admission check that was being performed is aborted; the `failurePolicy` for the ValidatingAdmissionPolicy

- Kube-apiserver: fix a possible panic validating a custom resource whose CustomResourceDefinition indicates a status subresource exists, but which does not define a `status` property in the `openAPIV3Schema` ([#135362](https://github.com/kubernetes/kubernetes/pull/135362), [@fusida](https://github.com/fusida)) [SIG API Machinery]

### API Change

- The metrics `etcd_request_duration_seconds` and `etcd_bookmark_counts` now differentiate by group resource instead of object type, allowing unique entries per `CustomResourceDefinition`, instead of grouping them all under `*unstructured.Unstructured`. ([#112042](https://github.com/kubernetes/kubernetes/pull/112042), [@ncdc](https://github.com/ncdc))