Share WordPress draft posts for client review without a plugin - drop in functions.php

wp-shareable-draft-preview.php

<?php
/**
 * Shareable draft preview for WordPress.
 *
 * Add ?key=YOUR_KEY to any post URL to preview drafts, pending,
 * and scheduled posts without logging in.
 *
 * Change the key value below to something unique and hard to guess.
 */
add_action('pre_get_posts', 'allow_draft_preview');
function allow_draft_preview($query) {
    if (is_admin() || !$query->is_main_query() || !$query->is_singular()) {
        return;
    }
    $key = isset($_GET['key']) ? sanitize_text_field(wp_unslash($_GET['key'])) : '';
    if (hash_equals('guestViewer_dBJ8vfaY', $key)) {
        $query->set('post_status', ['draft', 'pending', 'future']);
    }
}