The exit pop is not only available in the StoreFront StoreFronts which are using "Elements" theme at this time. It's only available within the legacy checkout. You can install the elements theme from the themes tab in the storefronts menu.

Please see the following for more details on adding a exit pop model to your storefront: Adding an Exit Intent Modal

(The problem with the exit pop these days is the body.unload fires for far more scenarios than them trying to close the browser (like the back button being used) and there is no way to differentiate the customers action. The feature that we recommend using which is more effective is the return email feature located under Operations → Marketing → Return Email. )

function exitPopOnLoad(ev) {
noCouponPop = false;
}
if (window.attachEvent) {
window.attachEvent('onload', exitPopOnLoad);
} else {
if (window.onload) {
var currentOnLoad = window.onload;
window.onload = function () {
currentOnLoad();
exitPopOnLoad();
};
} else {
window.onload = exitPopOnLoad;
}
}