8 files declare their use of StringItem

ComputedTestCacheableStringItem.php in core/modules/system/tests/modules/entity_test/src/Plugin/Field/FieldType/ComputedTestCacheableStringItem.php
InternalPropertyTestFieldItem.php in core/modules/system/tests/modules/entity_test/src/Plugin/Field/FieldType/InternalPropertyTestFieldItem.php
SingleInternalPropertyTestFieldItem.php in core/modules/system/tests/modules/entity_test/src/Plugin/Field/FieldType/SingleInternalPropertyTestFieldItem.php
StringFormatterTest.php in core/modules/field/tests/src/Unit/Plugin/Field/FieldFormatter/StringFormatterTest.php
StringItemTest.php in core/tests/Drupal/Tests/Core/Field/StringItemTest.php
StringNormalizer.php in core/modules/jsonapi/tests/modules/jsonapi_test_field_type/src/Normalizer/StringNormalizer.php
TimeZoneItem.php in core/modules/user/src/TimeZoneItem.php
UserNameItem.php in core/modules/user/src/UserNameItem.php

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