Checking if any key is up?

This may help you.

 bool holdingDown;
 
 void Update () {
 
         if (Input.anyKey) {
             Debug.Log("A key is being pressed");
             holdingDown = true;
         }
 
         if (!Input.anyKey && holdingDown) {
             Debug.Log("A key was released");
             holdingDown = false;
         }
 
 }

It doesn't work for individual keys, but close enough.

Could use the UnityGUI event calls in OnGUI()...

 private void OnGUI()
 {
     Event e = Event.current;
     if (e.type == EventType.KeyDown)
     {
         if (Input.GetKeyDown(e.keyCode))
         {
             Debug.Log("Down: " + e.keyCode);
         }
     }
     else if (e.type == EventType.keyUp)
     {
         if (Input.GetKeyUp(e.keyCode))
         {
             Debug.Log("Up: " + e.keyCode);
         }
     }
 }

More info: https://docs.unity3d.com/ScriptReference/Event.html

Since this is the first thing that comes up in a search, I figured I'd offer a solution since one doesn't seem to be provided by Unity.

The example will detect any key when it is pressed or held as well as detect when one of those buttons is released. This code also detects what specific keys are being pressed/released.

 public class GameInput : MonoBehaviour
 {
     protected List<KeyCode> m_activeInputs = new List<KeyCode>();
 
     public void Update()
     {
         List<KeyCode> pressedInput = new List<KeyCode>();
 
         if (Input.anyKeyDown || Input.anyKey)
         {
             foreach(KeyCode code in System.Enum.GetValues(typeof( KeyCode )) )
             {
                 if (Input.GetKey(code))
                 {
                     m_activeInputs.Remove( code );
                     m_activeInputs.Add( code );
                     pressedInput.Add( code );
 
                     Debug.Log( code +  " was pressed" );
                 }
             }
         }
 
         List<KeyCode> releasedInput = new List<KeyCode>();
 
         foreach(KeyCode code in m_activeInputs)
         {
             releasedInput.Add( code );
 
             if(!pressedInput.Contains(code))
             {
                 releasedInput.Remove( code );
 
                 Debug.Log( code +  " was released" );
             }
         }
 
         m_activeInputs = releasedInput;
     }
 }