window must be present for the page(frame) to work. Since window is declared in a global execution context, you can't redeclare it with let or const.

Although, it's possible to call the following code: var window = undefined; or window = undefined;, but it won't do anything to the window object. That's because the setter on the window object is undefined and configurable descriptor is set to false, disallowing changes and deletion of the window property. You can see that if you that with the following code:

console.log(Object.getOwnPropertyDescriptor(this, 'window'))

You are still able to declare your own window variable inside any other lexical context, other than the global one:

const test = () => {
  const window = 123;
  console.log('in test fn:', window);
}
test()
console.log('in global:', window)

But why can I change the globalThis then? You can change it because globalThis is the property of window. And doesn't seem to have an empty setter and configurable descriptor is set to true, allowing changes to this property. All that means, you can do anything you want with globalThis property :)

console.log(window.hasOwnProperty('globalThis'));
console.log(Object.getOwnPropertyDescriptor(window, 'globalThis'));