Disables WordPress Rest API for external requests

functions.php

/**
 * Disables WordPress Rest API for external requests
 *
 * @since  1.0.0
 * @access public
 *
 * @return void
 */
add_action( 'rest_api_init', function() {
    $whitelist = array('127.0.0.1', "::1");

    if(!in_array($_SERVER['REMOTE_ADDR'], $whitelist)){
        die('REST API is disabled.');
    }
}, 1 );