Just copy and paste following code snippet will set attribute - 'aria-hidden' for every <i> tag if it has classname 'fa' in your HTML code.

set-aria-hidden-i-tag.js

//for fontawesome icons (<i> tags with .fa)
(function(){
    let getIcons = document.querySelectorAll('i.fa');
    getIcons.forEach(function(iconEach)
    {
        let getIconAttr = iconEach.getAttribute('aria-hidden');
        if (!getIconAttr)
        {
            iconEach.setAttribute('aria-hidden','true');
        }
    });
})();