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

{
    //Note: needs to be called on the MCT
    var cimRenderer = featureLayer.GetRenderer() as CIMUniqueValueRenderer;
    //Gets the visual variables from the renderer
    var cimRotationVariable = cimRenderer.VisualVariables.OfType<CIMRotationVisualVariable>().FirstOrDefault();
    //Gets the visual variable info for the z dimension
    var rotationInfoZ = cimRotationVariable.VisualVariableInfoZ;
    //The Arcade expression used to evaluate and return the field name for the rotation
    var rotationExpression = rotationInfoZ.ValueExpressionInfo.Expression; // this expression stores the field name  
}

{
    // get the CIM renderer from the layer
    // Note: needs to be called on the MCT
    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  
}