Find WordPress posts that don't contain a specific search term.

wp_posts_without_keyword.php

<?php 
/* Get IDs of posts with content that doesn't contain a search term. */
$keyword      = 'search term here';
$max_posts    = 400; // More than the total number of posts.
$query_ids    = new WP_Query( array( 'post_type' => 'post', 'fields' => 'ids', 'posts_per_page' => $max_posts ) );
$query_search = new WP_Query( array( 'post_type' => 'post', 'fields' => 'ids', 'posts_per_page' => $max_posts, 's' => $keyword ) );

$ids_not_found = array_diff( $query_ids->posts, $query_search->posts );
sort( $ids_not_found );
print_r( $ids_not_found );

/*
// List found posts with edit and view links.
echo '<ol>';
foreach ( $ids_not_found as $post_id ) {
    echo "<li>{$post_id} <a href='/?p={$post_id}'>view</a> | <a href='/wp-admin/post.php?post={$post_id}&action=edit'>edit</a><br>";
}
echo '</ol>';
*/
?>