Twitter_Uncircle.user.js

// ==UserScript==
// @name         Twitter un-circle
// @namespace    https://gist.githubusercontent.com/DesertEagleDerek/afa3281b951b4b05c27422aa2ff104ee/raw/Twitter_Uncircle.user.js
// @updateURL    https://gist.githubusercontent.com/DesertEagleDerek/afa3281b951b4b05c27422aa2ff104ee/raw/Twitter_Uncircle.user.js
// @downloadURL  https://gist.githubusercontent.com/DesertEagleDerek/afa3281b951b4b05c27422aa2ff104ee/raw/Twitter_Uncircle.user.js
// @version      0.9
// @author       DesertEagleDerek
// @description  Revert to older square elements
// @homepage     http://www.deserteaglederek.com/
// @match        https://twitter.com/*
// @run-at       document-start
// @grant        none
// ==/UserScript==

function addGlobalStyle(css) {
    var head, style;
    head = document.getElementsByTagName('head')[0];
    if (!head) { return; }
    style = document.createElement('style');
    style.type = 'text/css';
    style.innerHTML = css;
    head.appendChild(style);
}

function addStyleCSS(values) {
    var finalCSS = "";
    for(var i = 0; i < values.length; i++) {
        for(var j = 0; j < values[i].length; j++) {
            if(j > 0) {
                finalCSS += values[i][j] + " !important;";
            } else {
                finalCSS += values[i][j] + " { ";
            }
        }
        finalCSS += " } \n";
    }
    addGlobalStyle(finalCSS);
}

(function() {
    'use strict';
    // array
    var cssArray = [];
    // get rid of those circles! (pre-2019 layout)
    cssArray.push([".Avatar, .ProfileAvatar-image", "border-radius:8px"]);
    cssArray.push([".ProfileAvatar", "border-radius:12px"]);
    cssArray.push([".ProfileCard-avatarLink", "border-radius:6px"]);
    cssArray.push([".ProfileCard-avatarLink img", "border-radius:5px"]);
    cssArray.push([".DashboardProfileCard-avatarImage", "border-radius:5px"]);
    cssArray.push([".avatar", "border-radius:5px"]);
    cssArray.push([".EdgeButton", "border-radius:5px"]);
    cssArray.push(["#tweet-box-global", "padding:5px"]);
    cssArray.push([".RichEditor", "border-radius:5px"]);
    cssArray.push([".tweet-content", "margin-bottom:16px"]);
    cssArray.push([".dropdown-toggle", "border-radius:8px"]);
    cssArray.push([".dropdown-toggle:before", "border-radius:8px"]);
    cssArray.push([".Gallery-media", "border-radius:6px"]);
    cssArray.push([".ProfileCardMini-avatarImage", "border-radius:4px"]);
    cssArray.push([".DMAvatar", "border-radius:4px"]);
    cssArray.push([".avatar--circular", "border-radius:4px"]);
    // get rid of those circles! (2019 layout)
    cssArray.push([".r-sdzlij", "border-radius:4px"]);
    // add style to page
    addStyleCSS(cssArray);
})();

Aha! "Document Body" seems to have fixed it for now. My "stress test" is to open 10+ new tabs (by, say, mass opening people's profiles from my geed) and then checking once loaded to see whether the elements are circles or squares. I will keep testing and see whether that remains the case. Thanks a lot!