8 files declare their use of PlainTextOutput in Zircon Profile 8

Attribute.php in core/lib/Drupal/Core/Template/Attribute.php

Contains \Drupal\Core\Template\Attribute.

ContactPersonalTest.php in core/modules/contact/src/Tests/ContactPersonalTest.php

Contains \Drupal\contact\Tests\ContactPersonalTest.

EmailAction.php in core/modules/action/src/Plugin/Action/EmailAction.php

Contains \Drupal\action\Plugin\Action\EmailAction.

FileItem.php in core/modules/file/src/Plugin/Field/FieldType/FileItem.php

Contains \Drupal\file\Plugin\Field\FieldType\FileItem.

MailManager.php in core/lib/Drupal/Core/Mail/MailManager.php

Contains \Drupal\Core\Mail\MailManager.

PlainTextOutputTest.php in core/tests/Drupal/Tests/Component/Render/PlainTextOutputTest.php

Contains \Drupal\Tests\Component\Render\PlainTextOutputTest.

system.module in core/modules/system/system.module

Configuration system that lets administrators modify the workings of the site.

user.module in core/modules/user/user.module

Enables the user registration and login system.