6 files declare their use of CategorizingPluginManagerInterface in Drupal 8

ActionManager.php in core/lib/Drupal/Core/Action/ActionManager.php

BlockManagerInterface.php in core/lib/Drupal/Core/Block/BlockManagerInterface.php

CategorizingPluginManagerTraitTest.php in core/tests/Drupal/Tests/Core/Plugin/CategorizingPluginManagerTraitTest.php

Contains \Drupal\Tests\Core\Plugin\CategorizingPluginManagerTraitTest.

ConditionManager.php in core/lib/Drupal/Core/Condition/ConditionManager.php

FieldTypePluginManagerInterface.php in core/lib/Drupal/Core/Field/FieldTypePluginManagerInterface.php

LayoutPluginManagerInterface.php in core/lib/Drupal/Core/Layout/LayoutPluginManagerInterface.php