Why does this script gives me a "synchronous XMLHttpRequest deprecated" error message?

Question

So I am testing a script at the moment and publish it with surge. Locally it works 100% of the time but whenever after publishing I get this error/warning message:

jquery-2.1.4.min.js [Deprecation] Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience."

I researched and found out that it has to do with async set to false. The script where its getting triggered below:

$(function(){
            $.getJSON('https://api.etherscan.io//api?module=stats&action=tokensupply&contractaddress=&apikey=4IEDV8D2CVTXVE2VFMUBSTUYVY827Q9GG1', function(data) {
            // Parse JSON file
             var myJSON = data;
             //Put result in var
             var string = myJSON.result;

             //Parse long string to right format
             const str = string;
             const res = str.slice(0, 3) + str.slice(3, 6);
                 //Apply result to html element
                    var elementSupply = document.getElementById("supply");
                    if (elementSupply) {
                        // elementSupply.innerHTML = res;
                        $("#supply").html(res);
                    }
                    if (document.getElementById("supply") === null) {
                        // elementSupply.innerHTML = res;
                        $("#supply").html(res);
                    }
         });
}); 

Nowhere is the 'async false' called. And I know that it is in this script because whenever I remove it, the warning message disappears.

Anyone has a clue? Thanks in advance.