6 files declare their use of MapDataDefinition in Drupal 9

ComplexDataConstraintValidatorTest.php in core/tests/Drupal/KernelTests/Core/TypedData/ComplexDataConstraintValidatorTest.php

LinkItem.php in core/modules/link/src/Plugin/Field/FieldType/LinkItem.php

MapDataNormalizerTest.php in core/modules/serialization/tests/src/Kernel/MapDataNormalizerTest.php

RecursiveContextualValidatorTest.php in core/tests/Drupal/Tests/Core/TypedData/RecursiveContextualValidatorTest.php

Contains \Drupal\Tests\Core\TypedData\RecursiveContextualValidatorTest.

TypedDataDefinitionTest.php in core/tests/Drupal/KernelTests/Core/TypedData/TypedDataDefinitionTest.php

TypedDataTest.php in core/tests/Drupal/KernelTests/Core/TypedData/TypedDataTest.php