Overview

Magento 2 Newsletter Subscription via Ajax.

Installation

1) Copy and paste the folder app/code/MDN into your /app/code/ folder.

2) Run the commands

php bin/magento setup:upgrade
php bin/magento cache:flush
php bin/magento setup:static-content:deploy 

Reference https://blog.mdnsolutions.com/magento-2-newsletter-subscription-via-ajax/