The 'Subscribe to newsletter popup' appears to unauthenticated users (users who are not logged in) after they read an article on your blog.
Its purpose is to encourage the readers to subscribe to your newsletter so they can get updates on your upcoming blog posts.
You have the option to keep this feature enabled or disabled from the Appearance settings section of your blog.
Follow the steps below to enable or disable the 'Subscribe to newsletter modal' on your blog:
Step 1: Navigate to the Appearance dashboard of your blog.
Step 2: Locate the "Subscribe to newsletter modal" section and enable or disable the 'Subscribe to newsletter modal' option according to your preference.
Step 3: Click the 'Update' button once you're done.