23 files declare their use of Inspector

ApcuBackend.php in core/lib/Drupal/Core/Cache/ApcuBackend.php
Cache.php in core/lib/Drupal/Core/Cache/Cache.php
CacheableNormalization.php in core/modules/jsonapi/src/Normalizer/Value/CacheableNormalization.php
CacheCollector.php in core/lib/Drupal/Core/Cache/CacheCollector.php
CacheTagsInvalidator.php in core/lib/Drupal/Core/Cache/CacheTagsInvalidator.php
Data.php in core/modules/jsonapi/src/JsonApiResource/Data.php
DatabaseBackend.php in core/lib/Drupal/Core/Cache/DatabaseBackend.php
DefaultPluginManager.php in core/lib/Drupal/Core/Plugin/DefaultPluginManager.php
EntityResource.php in core/modules/jsonapi/src/Controller/EntityResource.php
EntityResourceTestBase.php in core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
ErrorCollection.php in core/modules/jsonapi/src/JsonApiResource/ErrorCollection.php
IncludedData.php in core/modules/jsonapi/src/JsonApiResource/IncludedData.php
InspectorTest.php in core/tests/Drupal/Tests/Component/Assertion/InspectorTest.php
Contains \Drupal\Tests\Component\Assertion\InspectorTest.
Link.php in core/modules/jsonapi/src/JsonApiResource/Link.php
LinkCollection.php in core/modules/jsonapi/src/JsonApiResource/LinkCollection.php
MemoryBackend.php in core/lib/Drupal/Core/Cache/MemoryBackend.php
MemoryCache.php in core/lib/Drupal/Core/Cache/MemoryCache/MemoryCache.php
OmittedData.php in core/modules/jsonapi/src/JsonApiResource/OmittedData.php
PhpBackend.php in core/lib/Drupal/Core/Cache/PhpBackend.php
RelationshipData.php in core/modules/jsonapi/src/JsonApiResource/RelationshipData.php
ResourceObjectData.php in core/modules/jsonapi/src/JsonApiResource/ResourceObjectData.php
ResourceTypeBuildEvent.php in core/modules/jsonapi/src/ResourceType/ResourceTypeBuildEvent.php
ResourceTypeRepository.php in core/modules/jsonapi/src/ResourceType/ResourceTypeRepository.php

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