Notes

var jqueryScript = document.createElement('script');
jqueryScript.src = "//code.jquery.com/jquery-3.3.1.min.js";
jqueryScript.onload = function(){

// Important: change those text according to the text of the following button in your language
// e.g in Spanish it would be "Seguido"
let unfollowButtonText = "Following";
// Recently, a new confirmation dialog appears asking if you really want to unfollow the user
// change this text as well
// e.g in Spanish it would be "Dejar de seguir"
let unfollowConfirmationText = "Unfollow";

// Prepare jQuery Selector for buttons that contain "Following"
let selector = `button:contains('${unfollowButtonText}')`;

// You need to wait 60 seconds after every unfollow, otherwise you will
// be blocked temporary by the Instagram API and you'll see a 403 error in the network !
let currentTime = 0;
let step = 60 * 1000;

// Query the button
let unfollowButtons = $(selector);
// Total of buttons found
let totalUnfollowButtons = unfollowButtons.length;

if(!totalUnfollowButtons){
alert("Error: no Following buttons found, maybe change the text of the button?");
}

// Iterate on every button
unfollowButtons.each(function(index){
let button = $(this);

setTimeout(function(){
(function(i){
console.log(`Unfollowing ${i} of ${totalUnfollowButtons}`);

if(i == totalUnfollowButtons){
console.log("Script finished succesfully !");
}

button.trigger("click");

// Important: recently, a confirmation dialog was added when you click
// on unfollow, so simply click the confirmation button as well to unfollow the user
setTimeout(function(){
var btn = $(`button:contains('${unfollowConfirmationText}')`);

if(btn){
btn.trigger("click");
}
}, 100);
})(index + 1);
}, currentTime);

currentTime += step;
});
};

// Inject Script !
document.getElementsByTagName('head')[0].appendChild(jqueryScript);