Prevent camera approaching the player when it walks backwards? (CM_FreeLook)

You mean have something like a "minimum distance to target" on the vcam?

 

You can try adding a CinemachineExtension. Make sure it comes before CinemachineCollider if you want collider to still work.

Maybe this will work. Drop it into your scene and select it from the vcam's Extensions menu in the inspector.

Code (CSharp):

1. using UnityEngine;
2. using Cinemachine;
3.  
4. /// <summary>
5. /// An add-on module for Cinemachine Virtual Camera that clamps target disatance
6. /// </summary>
7. [ExecuteInEditMode] [SaveDuringPlay] [AddComponentMenu("")] // Hide in menu
8. public class ClampVcamDistance : CinemachineExtension
9. {
10.     [Tooltip("Clamp the vcam's distance from the target to at least this amount")]
11.     public float m_MinDistance = 10;
12.  
13.     private void OnValidate()
14.     {
15.         m_MinDistance = Mathf.Max(0, m_MinDistance);
16.     }
17.  
18.     protected override void PostPipelineStageCallback(
19.         CinemachineVirtualCameraBase vcam,
20.         CinemachineCore.Stage stage, ref CameraState state, float deltaTime)
21.     {
22.         if (stage == CinemachineCore.Stage.Body && VirtualCamera.Follow != null)
23.         {
24.             var dir = state.RawPosition - VirtualCamera.Follow.position;
25.             float d = dir.magnitude;
26.             if (d < m_MinDistance)
27.                 state.RawPosition = VirtualCamera.Follow.position + dir.normalized * m_MinDistance;
28.         }
29.     }
30. }
31.