20 files declare their use of PlainTextOutput

Attribute.php in core/lib/Drupal/Core/Template/Attribute.php
ConfigImporterMissingContentTest.php in core/tests/Drupal/KernelTests/Core/Config/ConfigImporterMissingContentTest.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
FinalExceptionSubscriber.php in core/lib/Drupal/Core/EventSubscriber/FinalExceptionSubscriber.php
MailManager.php in core/lib/Drupal/Core/Mail/MailManager.php
OEmbed.php in core/modules/media/src/Plugin/media/Source/OEmbed.php
OpenDialogCommand.php in core/lib/Drupal/Core/Ajax/OpenDialogCommand.php
PlainTextOutputTest.php in core/tests/Drupal/Tests/Component/Render/PlainTextOutputTest.php
RecipeCommand.php in core/lib/Drupal/Core/Recipe/RecipeCommand.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
Token.php in core/lib/Drupal/Core/Utility/Token.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.

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