6 files declare their use of NormalizerBase in Drupal 9

BooleanNormalizer.php in core/modules/serialization/tests/modules/test_datatype_boolean_emoji_normalizer/src/Normalizer/BooleanNormalizer.php

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

NormalizerBase.php in core/modules/hal/src/Normalizer/NormalizerBase.php

NormalizerBaseTest.php in core/modules/serialization/tests/src/Unit/Normalizer/NormalizerBaseTest.php

Contains \Drupal\Tests\serialization\Unit\Normalizer\NormalizerBaseTest.

StringNormalizer.php in core/modules/jsonapi/tests/modules/jsonapi_test_data_type/src/Normalizer/StringNormalizer.php

TraversableObjectNormalizer.php in core/modules/jsonapi/tests/modules/jsonapi_test_data_type/src/Normalizer/TraversableObjectNormalizer.php