/*
Plugin Name: Protect Shortcode Permalinks
Description: Restricts access to custom shortcode permalinks for logged-in users only.
*/

add_action('template_redirect', function () {
    $shortcode_name = get_query_var('shortcode_name');
    if ($shortcode_name && !is_user_logged_in()) {
        wp_redirect(home_url('/restricted-content'));
        exit;
    }
});
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<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /home2/nemoni61/public_html/wp-content/plugins/permalinks-authentication/permalinks-authentication.php:1) in <b>/home2/nemoni61/public_html/wp-includes/pluggable.php</b> on line <b>1435</b><br />
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<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /home2/nemoni61/public_html/wp-content/plugins/permalinks-authentication/permalinks-authentication.php:1) in <b>/home2/nemoni61/public_html/wp-includes/pluggable.php</b> on line <b>1438</b><br />