13 files declare their use of PluginFormInterface in Zircon Profile 8

ActionFormBase.php in core/modules/action/src/ActionFormBase.php

Contains \Drupal\action\ActionFormBase.

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

Contains \Drupal\aggregator\Plugin\AggregatorPluginSettingsBase.

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

Contains \Drupal\Core\Block\BlockPluginInterface.

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

Contains \Drupal\Core\Condition\ConditionInterface.

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

Contains \Drupal\Core\Action\ConfigurableActionBase.

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

Contains \Drupal\image\ConfigurableImageEffectInterface.

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

Contains \Drupal\search\Plugin\ConfigurableSearchPluginInterface.

ImageToolkitInterface.php in core/lib/Drupal/Core/ImageToolkit/ImageToolkitInterface.php

Contains \Drupal\Core\ImageToolkit\ImageToolkitInterface.

MenuLinkFormInterface.php in core/lib/Drupal/Core/Menu/Form/MenuLinkFormInterface.php

Contains \Drupal\Core\Menu\Form\MenuLinkFormInterface.

SearchPageFormBase.php in core/modules/search/src/Form/SearchPageFormBase.php

Contains \Drupal\search\Form\SearchPageFormBase.

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

Contains \Drupal\Core\Entity\EntityReferenceSelection\SelectionInterface.

SettingsForm.php in core/modules/aggregator/src/Form/SettingsForm.php

Contains \Drupal\aggregator\Form\SettingsForm.

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

Contains \Drupal\Core\Display\VariantInterface.