#ifs for new set of base classes

Assets/Scripts/EyePoseVisualizer.cs

     void OnEnable()
     {
         var faceManager = FindObjectOfType<ARFaceManager>();
-        if (faceManager != null && faceManager.subsystem != null && faceManager.subsystem.SubsystemDescriptor.supportsEyeTracking)
+        if (faceManager != null && faceManager.subsystem != null &&
+#if UNITY_2020_2_OR_NEWER
+            faceManager.subsystem.subsystemDescriptor.supportsEyeTracking)
+#else
+            faceManager.subsystem.SubsystemDescriptor.supportsEyeTracking)
+#endif
         {
             m_FaceSubsystem = (XRFaceSubsystem)faceManager.subsystem;
             SetVisible((m_Face.trackingState == TrackingState.Tracking) && (ARSession.state > ARSessionState.Ready));

Assets/Scripts/EyeTrackingUI.cs

         {
             m_Manager = FindObjectOfType<ARFaceManager>();
         }
-        if (m_Manager != null && m_Manager.subsystem != null && m_Manager.subsystem.SubsystemDescriptor.supportsEyeTracking)
+        if (m_Manager != null && m_Manager.subsystem != null &&
+#if UNITY_2020_2_OR_NEWER
+            m_Manager.subsystem.subsystemDescriptor.supportsEyeTracking)
+#else
+            m_Manager.subsystem.SubsystemDescriptor.supportsEyeTracking)
+#endif
         {
             var infoGO = GetComponent<Text>();
             infoGO.text = "This device supports eye tracking.";

Assets/Scripts/FixationPoint2DVisualizer.cs

     void OnEnable()
     {
         var faceManager = FindObjectOfType<ARFaceManager>();
-        if (faceManager != null && faceManager.subsystem != null && faceManager.subsystem.SubsystemDescriptor.supportsEyeTracking)
+        if (faceManager != null && faceManager.subsystem != null &&
+#if UNITY_2020_2_OR_NEWER
+            faceManager.subsystem.subsystemDescriptor.supportsEyeTracking)
+#else
+            faceManager.subsystem.SubsystemDescriptor.supportsEyeTracking)
+#endif
         {
             m_FaceSubsystem = (XRFaceSubsystem)faceManager.subsystem;
             SetVisible((m_Face.trackingState == TrackingState.Tracking) && (ARSession.state > ARSessionState.Ready));

Assets/Scripts/FixationPoint3DVisualizer.cs

     void OnEnable()
     {
         var faceManager = FindObjectOfType<ARFaceManager>();
-        if (faceManager != null && faceManager.subsystem != null && faceManager.subsystem.SubsystemDescriptor.supportsEyeTracking)
+        if (faceManager != null && faceManager.subsystem != null &&
+#if UNITY_2020_2_OR_NEWER
+            faceManager.subsystem.subsystemDescriptor.supportsEyeTracking)
+#else
+            faceManager.subsystem.SubsystemDescriptor.supportsEyeTracking)
+#endif
         {
             m_FaceSubsystem = (XRFaceSubsystem)faceManager.subsystem;
             m_Face.updated += OnUpdated;