29 files declare their use of Markup in Drupal 9

AssertHelperTraitTest.php in core/tests/Drupal/Tests/AssertHelperTraitTest.php

AttributeTest.php in core/tests/Drupal/Tests/Core/Template/AttributeTest.php

ConfigTest.php in core/tests/Drupal/Tests/Core/Config/ConfigTest.php

EngineTwigTest.php in core/modules/system/tests/src/Functional/Theme/EngineTwigTest.php

errors.inc in core/includes/errors.inc

Functions for error handling.

FieldPluginBaseTest.php in core/modules/views/tests/src/Unit/Plugin/field/FieldPluginBaseTest.php

Contains \Drupal\Tests\views\Unit\Plugin\field\FieldPluginBaseTest.

HtmlTag.php in core/lib/Drupal/Core/Render/Element/HtmlTag.php

HtmlTagTest.php in core/tests/Drupal/Tests/Core/Render/Element/HtmlTagTest.php

LinkGeneratorTest.php in core/tests/Drupal/Tests/Core/Utility/LinkGeneratorTest.php

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

MailTest.php in core/modules/system/tests/src/Kernel/Mail/MailTest.php

Messenger.php in core/lib/Drupal/Core/Messenger/Messenger.php

MessengerTest.php in core/tests/Drupal/KernelTests/Core/Messenger/MessengerTest.php

PluginBaseTest.php in core/modules/views/tests/src/Kernel/Plugin/PluginBaseTest.php

Contains \Drupal\views\Tests\Plugin\PluginBaseTest.

RendererPlaceholdersTest.php in core/tests/Drupal/Tests/Core/Render/RendererPlaceholdersTest.php

Contains \Drupal\Tests\Core\Render\RendererPlaceholdersTest.

RendererTest.php in core/tests/Drupal/Tests/Core/Render/RendererTest.php

Contains \Drupal\Tests\Core\Render\RendererTest.

SerializerTest.php in core/modules/jsonapi/tests/src/Kernel/Serializer/SerializerTest.php

SystemTestController.php in core/modules/system/tests/modules/system_test/src/Controller/SystemTestController.php

Test.php in core/modules/system/tests/modules/test_page_test/src/Controller/Test.php

TestController.php in core/modules/system/tests/modules/js_webassert_test/src/Controller/TestController.php

theme.inc in core/includes/theme.inc

The theme system, which controls the output of Drupal.

ThemeManager.php in core/lib/Drupal/Core/Theme/ThemeManager.php

ThemeRenderAndAutoescapeTest.php in core/tests/Drupal/KernelTests/Core/Theme/ThemeRenderAndAutoescapeTest.php

Contains \Drupal\KernelTests\Core\Theme\ThemeRenderAndAutoescapeTest.

TokenTest.php in core/tests/Drupal/Tests/Core/Utility/TokenTest.php

twig.engine in core/themes/engines/twig/twig.engine

Handles integration of Twig templates with the Drupal theme system.

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

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

TwigMarkupInterfaceTest.php in core/tests/Drupal/KernelTests/Core/Theme/TwigMarkupInterfaceTest.php

Contains \Drupal\KernelTests\Core\Theme\TwigMarkupInterfaceTest.

views.views.inc in core/modules/views/views.views.inc

Provide views data that isn't tied to any other module.