Is there an app, or way to get a notification if someone starts to type a text message? I know you can look at a text message and see the "..." if someone is responding, but I was curious if there is an app or way to get a notification for this event?
There is no app for that. On iOS there may never be one because there is no API access for that, but for the macOS desktop messages app, one could be made via hack-ish techniques like code injection.