6 files declare their use of CompositeConstraintBase in Drupal 9

CommentNameConstraint.php in core/modules/comment/src/Plugin/Validation/Constraint/CommentNameConstraint.php

EntityConstraintViolationList.php in core/lib/Drupal/Core/Entity/EntityConstraintViolationList.php

EntityTestCompositeConstraint.php in core/modules/system/tests/modules/entity_test/src/Plugin/Validation/Constraint/EntityTestCompositeConstraint.php

EntityValidationTest.php in core/tests/Drupal/KernelTests/Core/Entity/EntityValidationTest.php

MenuTreeHierarchyConstraint.php in core/modules/menu_link_content/src/Plugin/Validation/Constraint/MenuTreeHierarchyConstraint.php

TaxonomyTermHierarchyConstraint.php in core/modules/taxonomy/src/Plugin/Validation/Constraint/TaxonomyTermHierarchyConstraint.php