GUIText is deprecated, so what should I use instead of it?

The Text component replaced it. It's part of the new UI system called uGUI.

Go to GameObject ---> UI ---> Text and Unity will create a Canvas with a GameObject as a child. That child GameObject will have a Text component. You can learn more about the new UI system here.

Example of how to modify it from code:

public Text myText;

void Start()
{
    myText.text = "This is my text";
}

It is now replaced with the TextElement class in UnityEngine.UIElements

http://docs.unity3d.com/2021.2/Documentation/ScriptReference/UIElements.TextElement.html