When publishing a hosted feature service to a federated Portal for ArcGIS environment and the option, 'Prevent this item from being accidentally deleted' is selected, if the feature service is deleted from ArcGIS Server Manager, the associated hosted feature service in Portal for ArcGIS cannot be edited or deleted.
If you have delete protection enabled on a feature layer and you delete the associated Server service, the feature layer will remain in the Portal content. The delete option will be grayed out until you disable the delete protection on the item. Once this delete protection is disabled, then you are able to delete the layer from Portal.