A recreation of Drupal module Webform Protected Downloads (original can be found here) for Drupal 8.
The module has the following dependencies:
Clone the module into modules folder and enable it. Create a new Webform open up its settings (/admin/structure/webform/manage/{test_webform}/settings). You should notice a new setting Protected download files appears in the settings menu. Check the checkbox Enable serving protected files after webform submit, set expire time or leave blank for no expiration and check One time visit link. Finally upload a file you wish to serve.
There is a Webform protected download token available and can be used in form submission messages or sent via email.