public Polygon GetClipGeometry()

var map = MapView.Active.Map;
//If clipping is set to ArcGIS.Core.CIM.ClippingMode.None or ArcGIS.Core.CIM.ClippingMode.MapSeries null is returned
//If clipping is set to ArcGIS.Core.CIM.ClippingMode.MapExtent the ArcGIS.Core.CIM.CIMMap.CustomFullExtent is returned.
//Otherwise, if clipping is set to ArcGIS.Core.CIM.ClippingMode.CustomShape the custom clip polygon is returned.
var poly = map.GetClipGeometry();
//You can use the polygon returned
//For example: We make a polygon graphic element and add it to a Graphics Layer.
var gl = map.GetLayersAsFlattenedList().OfType<GraphicsLayer>().FirstOrDefault();
if (gl == null) return;
var polygonSymbol = SymbolFactory.Instance.ConstructPolygonSymbol(CIMColor.CreateRGBColor(255, 255, 0));
var cimGraphicElement = new CIMPolygonGraphic
{
  Polygon = poly,
  Symbol = polygonSymbol.MakeSymbolReference()
};
gl.AddElement(cimGraphicElement);