[WordPress] Simple way to protect REST API from the public

functions.php

/**
 * Author: Mattias Ghodsian
 * Donate a cup of coffee: https://www.buymeacoffee.com/mattiasghodsian
 * Donate Eth: 0xBBB96204E45D11C9799c6B12E6eE6F0d4A071Ef5
 *
 * custom_rest_api_authentication_token
 *
 * Protect REST API from the public | Generate token: https://bit.ly/2W5qDrB
 *
 * @param obj $wp_rest_server
 * @return json
 */
add_filter( 'rest_api_init', 'custom_rest_api_authentication_token', 99 );
function custom_rest_api_authentication_token($wp_rest_server){
    if ( !isset( $_REQUEST['token'] ) ) {
		return wp_send_json( ['code' => '401', 'message' => 'authentication token missing'] , 401);
		exit();
    }

    if ( $_REQUEST['token'] !== "token here") {
		return wp_send_json( ['code' => '403', 'message' => 'authentication token invalid'] , 403);
		exit();
    }
}