11 files declare their use of ImmutableConfig

AnnounceFetcher.php in core/modules/announcements_feed/src/AnnounceFetcher.php
ConfigEntityStorageTest.php in core/tests/Drupal/Tests/Core/Config/Entity/ConfigEntityStorageTest.php
CronSuspendQueueDelayTest.php in core/tests/Drupal/Tests/Core/Cron/CronSuspendQueueDelayTest.php
CronTest.php in core/tests/Drupal/Tests/Core/CronTest.php
DateTimeIso8601NormalizerTest.php in core/modules/serialization/tests/src/Unit/Normalizer/DateTimeIso8601NormalizerTest.php
DateTimeNormalizerTest.php in core/modules/serialization/tests/src/Unit/Normalizer/DateTimeNormalizerTest.php
HttpExceptionNormalizerTest.php in core/modules/jsonapi/tests/src/Unit/Normalizer/HttpExceptionNormalizerTest.php
ImmutableConfigTest.php in core/tests/Drupal/Tests/Core/Config/ImmutableConfigTest.php
PathProcessorFrontTest.php in core/tests/Drupal/Tests/Core/PathProcessor/PathProcessorFrontTest.php
UserRegistrationResource.php in core/modules/user/src/Plugin/rest/resource/UserRegistrationResource.php
UserRegistrationResourceTest.php in core/modules/user/tests/src/Unit/UserRegistrationResourceTest.php

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