Validate REST API requests by slug, and set proper response 404 status when not found.

get-post-by-slug.php

<?php declare(strict_types=1);

/**
 * Get Post By Slug
 * @wordpress-plugin
 * Plugin Name: Not Found Posts by Slug to 404
 * Description: Validate REST API requests by slug, and set proper response 404 status when not found.
 * Author: Austin Passy
 * Author URI: https://github.com/thefrosty
 * Version: 1.0.0
 * Requires at least: 5.2
 * Tested up to: 5.4.1
 * Requires PHP: 7.3
 * @package TheFrosty
 */

namespace TheFrosty;

\add_action('rest_api_init', static function (): void {

    /**
     * Validate the REST request when querying by key: `slug`.
     * WordPress returns proper 404's when no ID is found, but not when querying by slug.
     * @param \WP_REST_Response $response Result to send to the client. Usually a WP_REST_Response.
     * @param \WP_REST_Server $server Server instance.
     * @param \WP_REST_Request $request Request used to generate the response.
     * @return \WP_REST_Response
     */
    \add_filter('rest_post_dispatch', static function (
        \WP_REST_Response $response,
        \WP_REST_Server $server,
        \WP_REST_Request $request
    ): \WP_REST_Response {
        if (\array_key_exists('slug', $request->get_query_params()) &&
            !empty($request->get_param('slug')) &&
            $request->get_method() === \WP_REST_Server::READABLE &&
            empty($response->get_data())
        ) {
            $data = [
                'rest_post_invalid_slug',
                \esc_html__('Invalid post slug.', 'thefrosty'),
                ['status' => \WP_Http::NOT_FOUND],
            ];
            try {
                // Access the \WP_REST_Server::error_to_response method.
                $error_to_response = (new \ReflectionObject($server))->getMethod('error_to_response');
                $error_to_response->setAccessible(true);

                return $error_to_response->invoke($server, (new \WP_Error(...$data)));
            } catch (\ReflectionException $exception) {
                return new \WP_REST_Response($data, \WP_Http::NOT_FOUND);
            }
        }

        return $response;
    }, 10, 3);
});