26 files declare their use of FieldItemBase

BaseFieldDefinitionTest.php in core/tests/Drupal/Tests/Core/Entity/BaseFieldDefinitionTest.php
BooleanItem.php in core/lib/Drupal/Core/Field/Plugin/Field/FieldType/BooleanItem.php
CommentItem.php in core/modules/comment/src/Plugin/Field/FieldType/CommentItem.php
DateTimeIso8601NormalizerTest.php in core/modules/serialization/tests/src/Unit/Normalizer/DateTimeIso8601NormalizerTest.php
DateTimeItem.php in core/modules/datetime/src/Plugin/Field/FieldType/DateTimeItem.php
DummyAjaxItem.php in core/modules/image/tests/modules/image_module_test/src/Plugin/Field/FieldType/DummyAjaxItem.php
EmailItem.php in core/lib/Drupal/Core/Field/Plugin/Field/FieldType/EmailItem.php
EntityReferenceItem.php in core/lib/Drupal/Core/Field/Plugin/Field/FieldType/EntityReferenceItem.php
FieldTestItem.php in core/modules/system/tests/modules/entity_test/src/Plugin/Field/FieldType/FieldTestItem.php
LanguageItem.php in core/lib/Drupal/Core/Field/Plugin/Field/FieldType/LanguageItem.php
LayoutSectionItem.php in core/modules/layout_builder/src/Plugin/Field/FieldType/LayoutSectionItem.php
LinkItem.php in core/modules/link/src/Plugin/Field/FieldType/LinkItem.php
ListItemBase.php in core/modules/options/src/Plugin/Field/FieldType/ListItemBase.php
MapItem.php in core/lib/Drupal/Core/Field/Plugin/Field/FieldType/MapItem.php
MultiValueTestItem.php in core/modules/system/tests/modules/entity_test_update/src/Plugin/Field/FieldType/MultiValueTestItem.php
NumericItemBase.php in core/lib/Drupal/Core/Field/Plugin/Field/FieldType/NumericItemBase.php
PathItem.php in core/modules/path/src/Plugin/Field/FieldType/PathItem.php
SerializedItem.php in core/modules/system/tests/modules/entity_test/src/Plugin/Field/FieldType/SerializedItem.php
SerializedPropertyItem.php in core/modules/system/tests/modules/entity_test/src/Plugin/Field/FieldType/SerializedPropertyItem.php
ShapeItem.php in core/modules/system/tests/modules/entity_test/src/Plugin/Field/FieldType/ShapeItem.php
StringItemBase.php in core/lib/Drupal/Core/Field/Plugin/Field/FieldType/StringItemBase.php
TelephoneItem.php in core/modules/telephone/src/Plugin/Field/FieldType/TelephoneItem.php
TestItem.php in core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldType/TestItem.php
TestObjectItem.php in core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldType/TestObjectItem.php
TextItemBase.php in core/modules/text/src/Plugin/Field/FieldType/TextItemBase.php
TimestampItem.php in core/lib/Drupal/Core/Field/Plugin/Field/FieldType/TimestampItem.php

Buggy or inaccurate documentation? Please file an issue. Need support? Need help programming? Connect with the Drupal community.