It refers to https://github.com/kubernetes/enhancements/blob/master/keps/sig-api-machinery/20190425-structural-openapi.md

probably it's simpler than I thought, but we still have something wrong (probably in the CRD validation criteria) that causes it to be declared as NonStructuralSchema

NMO is delivered via CNV but all the engineering is happening in the telco/management team.
I will assign to Michael Moser once he creates an account here.

I'd suspect that the issue is just a missing `type: object` on the whole openAPIV3Schema:
https://github.com/kubevirt/node-maintenance-operator/blob/master/deploy/crds/nodemaintenance_crd.yaml#L16

but it comes from https://github.com/kubevirt/node-maintenance-operator/blob/master/pkg/apis/kubevirt/v1alpha1/zz_generated.openapi.go which was generated with openapi-gen

I have submitted a PR to solve this issue; it is pending review for quite some time. 
https://github.com/kubevirt/node-maintenance-operator/pull/70 - right now all comments have been addressed so that I don't know if there is a problem to merge it right now.

Thanks Michael, I didn't know there was a PR for this bug.
could you please update this BZ when the PR is merged?

The PR that is to solve the issue got merged now. https://github.com/kubevirt/node-maintenance-operator/pull/70

Verified on CNV-2.4.0, OCP: 4.5.0-rc.2, SSP build 58:
There are no validation errors:

$ oc get crd -n openshift-cnv nodemaintenances.nodemaintenance.kubevirt.io -oyaml
apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
  creationTimestamp: "2020-06-24T16:14:31Z"
  generation: 1
  managedFields:
  - apiVersion: apiextensions.k8s.io/v1beta1
    fieldsType: FieldsV1
    fieldsV1:
      f:spec:
        f:conversion:
          .: {}
          f:strategy: {}
        f:group: {}
        f:names:
          f:kind: {}
          f:listKind: {}
          f:plural: {}
          f:singular: {}
        f:preserveUnknownFields: {}
        f:scope: {}
        f:subresources:
          .: {}
          f:status: {}
        f:validation:
          .: {}
          f:openAPIV3Schema:
            .: {}
            f:properties:
              .: {}
              f:apiVersion:
                .: {}
                f:description: {}
                f:type: {}
              f:kind:
                .: {}
                f:description: {}
                f:type: {}
              f:metadata:
                .: {}
                f:type: {}
              f:spec:
                .: {}
                f:properties:
                  .: {}
                  f:nodeName:
                    .: {}
                    f:description: {}
                    f:type: {}
                  f:reason:
                    .: {}
                    f:description: {}
                    f:type: {}
                f:required: {}
                f:type: {}
              f:status:
                .: {}
                f:properties:
                  .: {}
                  f:evictionPods:
                    .: {}
                    f:description: {}
                    f:format: {}
                    f:type: {}
                  f:lastError:
                    .: {}
                    f:description: {}
                    f:type: {}
                  f:pendingPods:
                    .: {}
                    f:description: {}
                    f:items: {}
                    f:type: {}
                  f:phase:
                    .: {}
                    f:description: {}
                    f:type: {}
                  f:totalpods:
                    .: {}
                    f:description: {}
                    f:format: {}
                    f:type: {}
                f:type: {}
            f:type: {}
        f:version: {}
        f:versions: {}
      f:status:
        f:storedVersions: {}
    manager: catalog
    operation: Update
    time: "2020-06-24T16:14:31Z"
  - apiVersion: apiextensions.k8s.io/v1
    fieldsType: FieldsV1
    fieldsV1:
      f:status:
        f:acceptedNames:
          f:kind: {}
          f:listKind: {}
          f:plural: {}
          f:singular: {}
        f:conditions: {}
    manager: kube-apiserver
    operation: Update
    time: "2020-06-24T16:14:31Z"
  name: nodemaintenances.nodemaintenance.kubevirt.io
  resourceVersion: "39037"
  selfLink: /apis/apiextensions.k8s.io/v1/customresourcedefinitions/nodemaintenances.nodemaintenance.kubevirt.io
  uid: 3d7e36dc-6fbe-413c-8449-ed19acb785ac
spec:
  conversion:
    strategy: None
  group: nodemaintenance.kubevirt.io
  names:
    kind: NodeMaintenance
    listKind: NodeMaintenanceList
    plural: nodemaintenances
    singular: nodemaintenance
  preserveUnknownFields: true
  scope: Cluster
  versions:
  - name: v1beta1
    schema:
      openAPIV3Schema:
        properties:
          apiVersion:
            description: 'APIVersion defines the versioned schema of this representation
              of an object. Servers should convert recognized schemas to the latest
              internal value, and may reject unrecognized values. More info: https://git.k8s.io/community/contributors/devel/api-conventions.md#resources'
            type: string
          kind:
            description: 'Kind is a string value representing the REST resource this
              object represents. Servers may infer this from the endpoint the client
              submits requests to. Cannot be updated. In CamelCase. More info: https://git.k8s.io/community/contributors/devel/api-conventions.md#types-kinds'
            type: string
          metadata:
            type: object
          spec:
            properties:
              nodeName:
                description: Node name to apply maintanance on/off
                type: string
              reason:
                description: Reason for maintanance
                type: string
            required:
            - nodeName
            type: object
          status:
            properties:
              evictionPods:
                description: EvictionPods is the total number of pods up for eviction
                  from the start
                format: int64
                type: integer
              lastError:
                description: LastError represents the latest error if any in the latest
                  reconciliation
                type: string
              pendingPods:
                description: PendingPods is a list of pending pods for eviction
                items:
                  type: string
                type: array
              phase:
                description: Phase is the represtation of the maintenanace progress
                  (Running,Succeeded)
                type: string
              totalpods:
                description: TotalPods is the total number of all pods on the node
                  from the start
                format: int64
                type: integer
            type: object
        type: object
    served: true
    storage: true
    subresources:
      status: {}
status:
  acceptedNames:
    kind: NodeMaintenance
    listKind: NodeMaintenanceList
    plural: nodemaintenances
    singular: nodemaintenance
  conditions:
  - lastTransitionTime: "2020-06-24T16:14:31Z"
    message: no conflicts found
    reason: NoConflicts
    status: "True"
    type: NamesAccepted
  - lastTransitionTime: "2020-06-24T16:14:31Z"
    message: the initial names have been accepted
    reason: InitialNamesAccepted
    status: "True"
    type: Established
  storedVersions:
  - v1beta1

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHSA-2020:3194