Lichess now has the feature built-in! Enable it at the bottom of https://lichess.org/account/preferences/game-behavior.
// ==UserScript==
// @name Chess courtesy (deprecated, built-in to Lichess)
// @namespace http://tampermonkey.net/
// @version 1.2
// @description Lichess now has the feature built-in! Enable it at the bottom of https://lichess.org/account/preferences/game-behavior.
// @author lichess.org/@/thibault
// @include /^https://lichess\.org\/(\w{8}|\w{12})$/
// @grant none
// ==/UserScript==
(function() {
'use strict';
console.log("Lichess now has the feature built-in! Enable it at the bottom of https://lichess.org/account/preferences/game-behavior");
})();