17 files declare their use of ConfigurableInterface in Drupal 8

AggregatorPluginSettingsBase.php in core/modules/aggregator/src/Plugin/AggregatorPluginSettingsBase.php

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

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

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

ConfigurableSearchPluginInterface.php in core/modules/search/src/Plugin/ConfigurableSearchPluginInterface.php

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

Contains \Drupal\Tests\Core\Plugin\ContextHandlerTest.

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

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

FilterInterface.php in core/modules/filter/src/Plugin/FilterInterface.php

ImageEffectInterface.php in core/modules/image/src/ImageEffectInterface.php

LayoutBuilderEntityViewDisplay.php in core/modules/layout_builder/src/Entity/LayoutBuilderEntityViewDisplay.php

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

MediaSourceInterface.php in core/modules/media/src/MediaSourceInterface.php

SelectionPluginBase.php in core/lib/Drupal/Core/Entity/EntityReferenceSelection/SelectionPluginBase.php

TestConfigurablePlugin.php in core/tests/Drupal/Tests/Core/Plugin/Fixtures/TestConfigurablePlugin.php

VariantInterface.php in core/lib/Drupal/Core/Display/VariantInterface.php

WorkflowTypeInterface.php in core/modules/workflows/src/WorkflowTypeInterface.php