16 files declare their use of PlainTextOutput in Drupal 8

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

ContactPersonalTest.php in core/modules/contact/tests/src/Functional/ContactPersonalTest.php

EmailAction.php in core/lib/Drupal/Core/Action/Plugin/Action/EmailAction.php

EntityResourceValidationTrait.php in core/modules/rest/src/Plugin/rest/resource/EntityResourceValidationTrait.php

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

FileUpload.php in core/modules/jsonapi/src/Controller/FileUpload.php

FileUploadResource.php in core/modules/file/src/Plugin/rest/resource/FileUploadResource.php

FileUploadResourceTestBase.php in core/modules/rest/tests/src/Functional/FileUploadResourceTestBase.php

FileUploadTest.php in core/modules/jsonapi/tests/src/Functional/FileUploadTest.php

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

OpenDialogCommand.php in core/lib/Drupal/Core/Ajax/OpenDialogCommand.php

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

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

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

TemporaryJsonapiFileFieldUploader.php in core/modules/jsonapi/src/Controller/TemporaryJsonapiFileFieldUploader.php

UnprocessableHttpEntityExceptionNormalizer.php in core/modules/jsonapi/src/Normalizer/UnprocessableHttpEntityExceptionNormalizer.php

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

Enables the user registration and login system.