Currently, we are unable to edit the shutdown GUI so I think this should be implemented. We are able to edit the teleporting screen so why not the shutdown screen GUI? The current one ruins the theme and style of some games and by adding a custom shutdown GUI, developers can add GUI buttons that teleport the player to other games they have created or maybe a button that auto rejoins the player to the updated version of the game. This allows visual control and will be significant especially since developers have to shut down for every update.
At the very least, the current shutdown screen should be replaced with a simple plain GUI with a rejoin button.
Cheers!