public class CIMRotationVisualVariable : CIMVisualVariable, System.ComponentModel.INotifyPropertyChanged, System.Xml.Serialization.IXmlSerializable  

var featureLayer = MapView.Active.Map.GetLayersAsFlattenedList().OfType<FeatureLayer>().FirstOrDefault();
QueuedTask.Run(() =>
{
  var cimRenderer = featureLayer.GetRenderer() as CIMUniqueValueRenderer;
  var cimRotationVariable = cimRenderer.VisualVariables.OfType<CIMRotationVisualVariable>().FirstOrDefault();
  var rotationInfoZ = cimRotationVariable.VisualVariableInfoZ;
  var rotationExpression = rotationInfoZ.ValueExpressionInfo.Expression; // this expression stores the field name  
});

var featureLayer = MapView.Active.Map.GetLayersAsFlattenedList().OfType<FeatureLayer>().FirstOrDefault();
ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
{
  // get the CIM renderer from the layer
  var cimRenderer = featureLayer.GetRenderer() as ArcGIS.Core.CIM.CIMSimpleRenderer;
  // get the collection of connected attributes for rotation
  var cimRotationVariable = cimRenderer.VisualVariables.OfType<ArcGIS.Core.CIM.CIMRotationVisualVariable>().FirstOrDefault();
  // the z direction is describing the heading rotation
  var rotationInfoZ = cimRotationVariable.VisualVariableInfoZ;
  var rotationExpression = rotationInfoZ.Expression; // this expression stores the field name  
});