, why is xmlhttprequest synchronous deprecated?

I am having trouble understanding the ‘async’ part of XMLHttpRequest().
I have written a website www.weemfg.com using javascript and PHP using
syncronous http which works fine using firefox however when i use chrome
i get warnings about my use of ‘synchronous’ being deprecated. I want
the data returned to my call of majax below. using async it will go
to someplace else and how will i know when it is there. allowing timeout
to synchronous xhttp might solve my problem.

what i want … let rsp = majax(“qry,”select * from sometable”).

function majax(op, argv) {
    /* ------------------------------------------------------------------
      this is synchronous. Since the user has initiated communication
      and this is a single purpose app there is no need for 'async'
      there will always be a rsp[0]
    ------------------------------------------------------------------- */
    //console.log("at majax op="+op+" argv="+argv);
    let pdata = new FormData();
    pdata.append('op', op);
    pdata.append('argv', argv);

    let rsp = false;
    let xhr = new XMLHttpRequest();
    xhr.open('POST', 'mfg.php', false);     // false=synchronous
    xhr.onload = function () {
        rsp = this.responseText;
    xhr.send(pdata);              // this wont return until onload finishes
    return rsp;

i have tried to accomplish what i want using async without success.