public void SetVisibility( 
   bool isVisible
)

{
    if (!layer.IsVisible)
        layer.SetVisibility(true);

    if (layer is FeatureLayer featureLayerToChange)
    {
        if (!featureLayer.IsEditable)
            featureLayer.SetEditable(true);

        if (!featureLayer.IsSnappable)
            featureLayer.SetSnappable(true);
    }
}

{
  // Note: call within QueuedTask.Run()
  {
    var url = "https://geoeventsample1.esri.com:6443/arcgis/rest/services/AirportTraffics/StreamServer";
    var createParam = new FeatureLayerCreationParams(new Uri(url))
    {
      IsVisible = false //turned off by default
    };
    streamLayer = LayerFactory.Instance.CreateLayer<StreamLayer>(createParam, map);

    //or use "original" create layer (will be visible by default)
    Uri uri = new Uri(url);
    streamLayer = LayerFactory.Instance.CreateLayer(uri, map) as StreamLayer;
    streamLayer.SetVisibility(false);//turn off
  }
}