I added a script on greasemonkey for HN visited linked with : I dont remember where did I get that code, but it work for me.

    // ==UserScript==
    // @name     HN visited link
    // @version  1
    // @grant    all
    // ==/UserScript==
    
    let aCss = `
        .news-list a:visited {
    			color: lightgray;
        }
    `
    
    addGlobalStyle(aCss);
    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);
    }