The development of this plugin by Sylius organization is currently frozen. We may or may not approach the problem again but nothing is planned yet. Please use the production-ready plugin by BitBag available here: https://github.com/BitBagCommerce/SyliusElasticsearchPlugin.
If you would like to fork this plugin and continue its development, please consider contributing to the plugin mentioned above. If you still think it is a good idea to fork, then feel free to do so when respecting the MIT license.
Thanks for your understanding and happy coding! :)