Remove for everyone all messages from a current conversation in Messenger (updated, 29.12.2020.)

// Copy everything from official JQuery script (https://code.jquery.com/jquery-3.5.1.slim.min.js), paste it in developer console (open it up with right click on page and "Inspect element") and hit Enter, thereafter you can execute the following:
// Warning: Do not hover over your messages with cursor during the execution of this script!

function r() {
    $('.buofh1pr.j83agx80.eg9m0zos.ni8dbmo4.cbu4d94t.gok29vw1').scrollTop(0)
    setTimeout(function w(){
        if ($('.r7fvqmod.ee40wjg4.ega7oslk.stwyh623.pf5603km.ev399l9o.lzcic4wl').length) setTimeout(w, 500); else f()
    }, 1500)
}
var f = function () {
    do {
        var o = o ? o.parent().parent().parent().parent().prevAll() : $(document)
        o = o.find('.pfnyh3mw.ewlkfwdl.j83agx80.bp9cbjyn.qnrpqo6b.jxrgncrl')
        if (!o.length) return !$('.aodizinl.d1544ag0.osnr6wyh.tw6a2znq').length ? r() : false
    } while (o.last().prev().find('.erlsw9ld.fmqa7x8m.qv66sw1b').length);
    o.last()[0].scrollIntoView()
    o.last()[0].dispatchEvent(new MouseEvent('mouseover', {bubbles: true}))
    setTimeout(function (){
        var m = $('.mkd47r93.rgmg9uty.j83agx80.rl25f0pe .p1ueia1e.pgctjfs5.l8rlqa9s.sh06z9xi.i2p6rm4e')
        o.last()[0].dispatchEvent(new MouseEvent('mouseout', {bubbles: true}))
        if (m.length === 1) return setTimeout(f, 200)
        m.last().click()
        setTimeout(function (){
            $('.l9j0dhe7.ama3r5zh > div > div > div > div:first').click()
            setTimeout(function () {
                $('.ll8tlv6m.j83agx80.taijpn5t.hzruof5a > div > div:last > div > div:first > div:first').click()
                setTimeout(f, 1500)
            }, 200)
        }, 100)
    }, 100)
}
f()