10 files declare their use of PluralTranslatableMarkup in Drupal 9

LocalePluralFormatTest.php in core/modules/locale/tests/src/Functional/LocalePluralFormatTest.php

MigrateUpgradeImportBatch.php in core/modules/migrate_drupal_ui/src/Batch/MigrateUpgradeImportBatch.php

NumericField.php in core/modules/views/src/Plugin/views/field/NumericField.php

PermissionHandlerTest.php in core/modules/user/tests/src/Unit/PermissionHandlerTest.php

Contains \Drupal\Tests\user\Unit\PermissionHandlerTest.

PluralTranslatableMarkupTest.php in core/tests/Drupal/Tests/Core/StringTranslation/PluralTranslatableMarkupTest.php

StatusReportPage.php in core/modules/system/src/Element/StatusReportPage.php

StringTranslationTraitTest.php in core/tests/Drupal/Tests/Core/StringTranslation/StringTranslationTraitTest.php

system.install in core/modules/system/system.install

Install, update and uninstall functions for the system module.

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

user.post_update.php in core/modules/user/user.post_update.php

Post update functions for User module.