Is there a shortcut to close all windows on an iPad?

Apps can be forced closed from the App Switcher.

Swipe upwards from the bottom edge of the screen to expose the Task Switcher - or for iPad models with a physical Home Button, double click. Each open App can be seen, the most recently accessed App is always to the right of the screen; Apps are individually force-closed by swiping each App off the top of the screen.

There isn’t a single gesture to close all the Apps.

It is perhaps worth pointing out that most Apps visible within the Task Switcher will be “inactive” - meaning that they are consuming negligible system resources. The App window that you see merely shows the currently “saved state” of the App.

I hope this information and insight proves to be helpful.