Show comment author e-mail in comment.html.twig
If you have a fresh Drupal 8 standard install, go to: 'People >
  Permissions' and Allow anonymous users to 'Post comments' and 'Skip
  comment approval'. Then go to: 'Structure > Content Types > Article >
  Manage Fields' and edit the 'Comments' field. Go to 'Anonymous
  commenting' and select 'Anonymous posters may leave their contact
  information'. Click 'Save settings'. Now when you go post a comment as
  an anonymous user you can leave your e-mail.


You can print that e-mail value in plain text like this:

{{ content.comment_body['#object'].getAuthorEmail() }}


Now, this will print the e-mails of logged in users as well. If you don't want that, then you will need to add a conditional to check if their User ID is 0, which means they are anonymous.

{% if attributes.toArray()['data-comment-user-id'] == 0 %}

    {{ content.comment_body['#object'].getAuthorEmail() }}

{% endif %}