Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience

Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience

how can browser console log warning : "Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience" ?

My jquery version

/*!
 * jQuery JavaScript Library v3.4.1

My ajax script

$.ajax({ 
 url: '$url', 
 type: 'post', 
 // dataType: 'json', 
 // data: data, })
 .done(function(response) {
       $("#inject").html(response); 
 }) .fail(function(xhr) { 
       console.log(xhr); 
 });