How to freeze enemy movement when Hurt/enemy Damage?

It looks like youre using root motion and mecanim to drive the zombies movement? Depending on what your hurt animation is actually animating is determining the zombies movement speed. if you want the zombie to pause longer in the hurt state your animation should actually include that pause.

if youre trying to get the zombie to pause after every hurt but dont have the means to modify/add a new hurt animation you could insert a new animation to follow up the hurt animation and then transition from that new animation to the other states. you could try putting another "Zombie Idle" animation into your mecanim state machine and name it something like "pause". have "Hurt1" transition only to that state then reconnect the new "pause" state to zombie idle. The transition from pause -> idle should have "Has Exit Time" checked and you would set the "Exit Time" setting to however long you want that pause to play.

https://i.imgur.com/DV6GOCW.png

If this is not what you meant and I misunderstood then maybe youre looking to stop the zombie from rotating? In order to do this you need to know when the zombie is actually hurt. I added some things to your code and commented my additions, maybe it will help.

NewEnemyScript:

 public class NewEnemyScript : MonoBehaviour
 {
     //use holster master as the player target
     [SerializeField] private Transform playerTarget;
     [SerializeField] private float attackRange;
     [SerializeField] private float alertRange;
     [SerializeField] private float moveSpeed;
     [SerializeField] private Animator anim;
 
     //need a reference to the health script to determine if the zombie is dead or hurt
     [SerializeField] private NewHealth health;
 
     void Update()
     {
         //if we are hurt or dead just return and dont run the code below
         if (health.IsHurt || health.IsDead) { return; }
 
 
         if (Vector3.Distance(playerTarget.position, this.transform.position) < alertRange)
         {
             Vector3 direction = playerTarget.position - this.transform.position;
             direction.y = 0;
 
             this.transform.rotation = Quaternion.Slerp(this.transform.rotation, Quaternion.LookRotation(direction), 0.1f);
             anim.SetBool("isIdle", false);
             if (direction.magnitude > attackRange)
             {
                 this.transform.Translate(0, 0, moveSpeed);
                 anim.SetBool("isRunning", true);
                 anim.SetBool("isAttack", false);
             }
             else
             {
                 anim.SetBool("isAttack", true);
                 anim.SetBool("isRunning", false);
             }
         }
         else
         {
             anim.SetBool("isIdle", true);
             anim.SetBool("isAttack", false);
             anim.SetBool("isRunning", false);
         }
     }
 }

NewHealth:

 public class NewHealth : MonoBehaviour
 {
     [SerializeField] private float enemyHealth = 100f;
     [SerializeField] private Animator animator;
     [SerializeField] private float deductHealth = 10f;
     //the time that the zombie will pause after being hurt (will not rotate during this time)
     [SerializeField] private float hurtPauseTime = 1f;
 
 
     private bool _isHurt = false;
     private bool _isDead = false;
 
     //cached reference to the coroutine so we can start/stop it whenever we want
     Coroutine hurtRoutine;
 
     //public bool used by "newenemyscript" to determine if we are dead
     public bool IsDead
     {
         get
         {
             return _isDead;
         }
     }
     //public bool used by "newenemyscript" to determine if we are hurt
     public bool IsHurt
     {
         get
         {
             return _isHurt;
         }
     }
     
 
     void Update()
     {
         if (!_isDead)
         {
             if (enemyHealth <= 0)
             {
                 enemyDead();
 
             }
         }
     }
 
     public void DeductHealth()
     {
         if (!_isDead)
         {
             enemyHealth -= deductHealth;
             animator.SetTrigger("Hurt");
 
             //if we are already hurt lets stop the coroutine so that multiple coroutines arent running
             if (_isHurt)
             {
                 StopCoroutine(hurtRoutine);
             }
             //start the coroutine and store a reference to it so that we can stop it whenever we want
             hurtRoutine = StartCoroutine(HurtWait(hurtPauseTime));
         }
     }
 
     void enemyDead()
     {
         Destroy(gameObject, 5);
         animator.SetBool("isDead", true);
         _isDead = true;
     }
 
     private void OnTriggerEnter(Collider other)
     {
         if (other.gameObject.tag == "Bullet")
         {
             DeductHealth();
             Destroy(other.gameObject);
             Debug.Log("Hurt");
         }
     }
 
     //enable isHurt, pause for "waitTime" in seconds, disable isHurt
     IEnumerator HurtWait(float waitTime)
     {
         _isHurt = true;
         yield return new WaitForSeconds(waitTime);
         _isHurt = false;
     }
 }