Enables https://mywebsite.com/@username for author URLs in WordPress

wp_custom_author_url_rewrite.php

<?php
// Enables https://mywebsite.com/@username for author URLs
add_action('init', 'custom_author_url_rewrite');
function custom_author_url_rewrite() {
  add_rewrite_rule('^@([a-zA-Z0-9.-_]+)/?', 'index.php?author_name=$matches[1]', 'top');
}

// Changes get_author_posts_url() response to https://mywebsite.com/@username author URLs
add_filter('author_link', 'custom_author_url_filter', 10, 3);
function custom_author_url_filter($link, $author_id, $author_nicename) {
  $user = get_user_by('id', $author_id);
  return network_home_url("/@{$user->user_login}");
}
?>