espocrm/application/Espo/Modules/Crm/Resources/metadata/logicDefs/Task.json

{
    "fields": {
        "dateCompleted": {
            "visible": {
                "conditionGroup": [
                    {
                        "type": "equals",
                        "attribute": "status",
                        "value": "Completed"
                    }
                ]
            }
        }
    }
}