public CIMVisualVariable[] VisualVariables {get; set;}

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  
});