8 files declare their use of ConfigurablePluginBase

ConfigurablePluginBaseTest.php in core/tests/Drupal/Tests/Core/Plugin/ConfigurablePluginBaseTest.php
DefaultSingleLazyPluginCollectionTest.php in core/tests/Drupal/Tests/Core/Plugin/DefaultSingleLazyPluginCollectionTest.php
ImageEffectBase.php in core/modules/image/src/ImageEffectBase.php
LayoutDefault.php in core/lib/Drupal/Core/Layout/LayoutDefault.php
SelectionPluginBase.php in core/lib/Drupal/Core/Entity/EntityReferenceSelection/SelectionPluginBase.php
TestConfigurablePlugin.php in core/tests/Drupal/Tests/Core/Plugin/Fixtures/TestConfigurablePlugin.php
VariantBase.php in core/lib/Drupal/Core/Display/VariantBase.php
WorkflowTypeBase.php in core/modules/workflows/src/Plugin/WorkflowTypeBase.php

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