How to disable a keypress after a certain amount of time?

WaitForSeconds will pause the coroutine, you can use WaitForEndOfFrame and Time.deltaTime to keep track of time progress. Can try something like this:

 bool boostAvailable = true;
 
 IEnumerator SpeedUpBoost () {
     updateOn = false;
     boostAvailable = false;
 
     float elapsed = 0f;
     while (elapsed < speedUpCutoffTime)
     {
         rigbody.velocity = new Vector2 (speed, 40);
         elapsed += Time.deltaTime;
         yield return new WaitForEndOfFrame();
     }

     updateOn = true;
 
     // simple cooldown, prevent boost for some time
     // if an item makes the boost available, then it can set boostAvailable to true instead.
     yield return new WaitForSeconds(5f);
     boostAvailable = true;
 }
 
 
 // Update is called once per frame
 void Update () {
     if (updateOn == true) {
         if (Input.GetKey (moveUp)) {
             rigbody.velocity = new Vector2 (speed, 20);
         } else if (Input.GetKey(speedUp) && boostAvailable) {
             StartCoroutine(SpeedUpBoost());
         } else if (Input.GetKey (speedDown)) {
             rigbody.velocity = new Vector2 (speed, -20);
         } else {
             rigbody.velocity = new Vector2 (speed, 0);
         }
     }
 }