FramingTransposer at fixed height

You could try making a custom extension that forces the camera Y to a fixed value, maybe like this:

Code (CSharp):

1. using UnityEngine;
2. using Cinemachine;
3.  
4. /// <summary>
5. /// An add-on module for Cinemachine Virtual Camera that locks the camera's Y co-ordinate
6. /// </summary>
7. [SaveDuringPlay] [AddComponentMenu("")] // Hide in menu
8. public class LockCameraY : CinemachineExtension
9. {
10.     [Tooltip("Lock the camera's Y position to this value")]
11.     public float m_YPosition = 10;
12.  
13.     protected override void PostPipelineStageCallback(
14.         CinemachineVirtualCameraBase vcam,
15.         CinemachineCore.Stage stage, ref CameraState state, float deltaTime)
16.     {
17.         if (stage == CinemachineCore.Stage.Body)
18.         {
19.             var pos = state.RawPosition;
20.             pos.y = m_YPosition;
21.             state.RawPosition = pos;
22.         }
23.     }
24. }
25.  

Drop it into your project, it will appear automatically in the vcam's Extensions dropdown.