Simple MU plugin to prevent WordPress REST API access without login

wp-rest-api-auth.php

<?php
/**
 * Plugin Name: WP REST API Login Filter
 * Plugin URI: https://skyland.dev
 * Description: Disables the WP rest API without login.
 * Version: 1
 * Author: Sky
 * Author URI: https://skyland.dev
 *
 */

add_filter('rest_authentication_errors', function ($result) {
    // If a previous authentication check was applied,
    // pass that result along without modification.
    if (true === $result || is_wp_error($result)) {
        return $result;
    }

    // No authentication has been performed yet.
    // Return an error if user is not logged in.
    if (!is_user_logged_in()) {
        return new WP_Error(
            'rest_not_logged_in',
            __('No way! You are not currently logged in.'),
            array('status' => 401)
        );
    }

    // Our custom authentication check should have no effect
    // on logged-in requests
    return $result;
});