Hi @degaray ,
you can currently subscribe/unsubscribe customers to the newsletter via the Customer resource. You PUT something like the following, specifying the extension_attributeis_subscribed:
There's an issue currently reading this extension attribute on GET. The problem is that the plugin for the Customer module does not express how to populate this newsletter extension attribute data on a GET of a customer resource, hence it's missing from the response. The missing function needs to be defined here:
<magento dir>/app/code/Magento/Newsletter/Model/Plugin/CustomerPlugin.php