A really simple plugin to add a second parameter to the author URL

rewrite.php

<?php

/**
 * Plugin Name:       Rewrite Author URL
 * Plugin URI:        https://example.com/plugins/the-basics/
 * Description:       A basic rewrite rules example.
 * Version:           1.0.0
 * Requires at least: 5.2
 * Requires PHP:      7.4
 * Author:            Enrique Chavez
 * Author URI:        https://enriquechavez.co/
 */


add_filter('author_rewrite_rules', function ($author_rules) {
    // Add a new regex to match the following URL structure author/name/metafield
    // And assign the result to the metafield variable
    $author_rules['author/([^/]+)/([^/]+)?$'] = 'index.php?author_name=$matches[1]&metafield=$matches[2]';

    return $author_rules;
});

add_filter('query_vars', function ($vars) {
    // Add the new var to be able to handle it using the WordPress way!
    $vars[] = 'metafield';

    return $vars;
});

add_shortcode('show_user_metafield', function ($atts) {
    // Simple shortcode to show the new field value
    // This should only be parsed on the author page
    if ( ! is_author()) {
        return '';
    }

    // Get the variable value
    $metafield = get_query_var('metafield');

    // If the metafield is empty do nothing.
    if (empty($metafield)) {
        return '';
    }

    // Do whatever you want with the value.
    return '<div>Mostrando el meta field: ' . $metafield . '</div>';
});