Set cache headers on WordPress 404 pages.

functions.php

<?php 
/**
 * Force cache headers on 404 pages and prevent WordPress from handling 404s.
 *
 * @param bool $preempt determines who handles 404s.
 * @param obj  $wp_query global query object.
 */
function change_404_headers( $preempt, $wp_query ) {
	if ( ! is_admin() && ! is_robots() && count( $wp_query->posts ) < 1 ) {
		header( 'Cache-Control: max-age=30000, must-revalidate' );
		header( 'Expires: ' . gmdate( 'D, d M Y H:i:s', strtotime( '+5000 minutes' ) ) . ' GMT' );
		header( 'Last-Modified: ' . gmdate( 'D, d M Y H:i:s', strtotime( '-5000 minutes' ) ) . ' GMT' );
		$wp_query->set_404();
		status_header( 404 );
		// prevents the default 404 from firing.
		return true;
	}
	return $preempt;
}
add_filter( 'pre_handle_404', 'change_404_headers', 2, 99 );

Please note, this has uncovered some unexpected behavior in 4.9 and should not be used. A bug has been filed to get to the bottom of things. I'll update this gist as the ticket progresses.

The issue discovered is when the site is using a permalink structure where the first variable is one of: postname, category, tag, or author. This throws WP_Rewrite into 'use_verbose_page_rules' mode. This means that WP_Query is always in a state of error because no rewrite rule matches but the query still runs and performs a standard blog query where the most recent posts are returned but the query still calls $wp_query->set_404. This causes the count( $wp_query->posts ) < 1 check to fail.

Also, the action and argument count arguments are inverted.

Is there any update on the issue? @MikeNGarrett @kshaner

@slim16165 Here's something that I've been trying out today and seems to work so far:

add_action('nocache_headers', function($headers){
    if (is_404() ) {
        /// Unset the WordPress "nocache" headers    
        unset( $headers['Expires'] );
        unset( $headers['Cache-Control'] );
        // Alternatively set new headers
        $headers['Cache-Control'] = 'max-age=300';
    }
    return $headers;
});