Updated June 10th, 2019.
Apple shows you “Software Update” notifications at the top right of your screen, and it never seems to happen when it’s convenient for you! If you click “Details” you have to wait for a screen to load. If you click “Update” you have to wait for the update to install. If you see a “Later” button and you click it you’ll end up installing it automatically, when maybe you really don’t want to.
If all you want to do is make the box go away just click and drag it to the right.
Do that and the notification will simply go away. Yes, it will come back. But not right away.