code: func DropDisabledFields(pvSpec *api.PersistentVolumeSpec, oldPVSpec *api.PersistentVolumeSpec) {
  	if !utilfeature.DefaultFeatureGate.Enabled(features.BlockVolume) && !volumeModeInUse(oldPVSpec) {
  		pvSpec.VolumeMode = nil
  	}
  }
docs: // DropDisabledFields removes disabled fields from the pv spec.
  // This should be called from PrepareForCreate/PrepareForUpdate for all resources containing a pv spec.