declutter-twitter.user.js

// ==UserScript==
// @name           Declutter Twitter
// @namespace      https://titanous.com
// @description    Removes "Who To Follow", "Trends" and the legalese/footer
// @include        http://twitter.com/*
// @include        http://www.twitter.com/*
// @include        https://twitter.com/*
// @include        https://www.twitter.com/*
// @grant          none
// ==/UserScript==

var style = document.createElement('style');
style.type = 'text/css';
var css = "div.component[data-component-term='user_recommendations'], " +
	        "div.component[data-component-term='trends'], " +
	        "div.component[data-component-term='footer'], " +
	        "div.recent-followers-module, " +
	        "div.promoted-tweet, " + 
	        "div.module.trends," + 
	        "div.promptbird," +
	        "div.wtf-module, " +
	        "div.site-footer, " +
	        "div.Footer, " +
	        "div.ScrollBump--recentlyFollowed, " +
	        "div.ProfileWTFAndTrends, " +
	        "div#js-empty-timeline-recommendations-module-hook " +
	        "{ display: none !important; }";
style.appendChild(document.createTextNode(css));
document.getElementsByTagName("head")[0].appendChild(style);

After installing the script and reloading the main page, twitter.com didn't notice any difference.
Mozilla Firefox 99.0.1 (64-bit), Tampermonkey 4.16.6160 (6 April 2022).