Make my mathf to Wait? :)

Time.time is continuously increased and PingPong directly depends on the passed value. So if you want to delay the change you have to delay / stop the increasing of the value that you use for ping pong. So you can't use Time.time. You can use your own "timer" variable

 bool timerActive = true;
 float timer = 0;
 
 public void Update()
 {
     if (timerActive)
         timer += Time.deltaTime;
     platform.GetComponent<Ferr2DT_PathTerrain> ().vertexColor.a = Mathf.PingPong(timer,1.25f);
 }

When you set timerActive to false the pingpong will basically freeze until you set it back to true.

Note that you have a period of 1.25 so you set your alpha value up to 1.25. The usual range is 0-1

I guess this should do it:

 IEnumerator YourFunc() {
     for (;;) {
         float t = Time.time;
         platform.GetComponent<Ferr2DT_PathTerrain> ().vertexColor.a =Mathf.PingPong(t,1.25f);
     
         yield return new WaitForSeconds(1f);
     }
 }

And to start the coroutine you have to call StartCoroutine("YourFunc"). If you need to do it untill certain condition is met - just replace (;;) with the condition you need.