Keep old permalinks

/**
 * Force original permalinks for old products
 */
function pm_keep_old_permalinks($permalink, $post, $old_permalink) {
    // Ignore new permalinks
    if(strtotime($post->post_date) > strtotime('2020-03-15')) {
        return $permalink;
    }

    // Filter only product permalinks
    if(empty($post->post_type) || $post->post_type !== 'product') {
        return $permalink;
    }

    return $old_permalink;
}
add_filter('permalink_manager_filter_final_post_permalink', 'pm_keep_old_permalinks', 999, 3);