21 files declare their use of BlockPluginInterface in Drupal 8

BlockComponentRenderArray.php in core/modules/layout_builder/src/EventSubscriber/BlockComponentRenderArray.php

BlockComponentRenderArrayTest.php in core/modules/layout_builder/tests/src/Unit/BlockComponentRenderArrayTest.php

BlockEntitySettingTrayForm.php in core/modules/settings_tray/src/Block/BlockEntitySettingTrayForm.php

BlockForm.php in core/modules/block/src/BlockForm.php

BlockInterface.php in core/modules/block/src/BlockInterface.php

BlockInterfaceTest.php in core/modules/block/tests/src/Kernel/BlockInterfaceTest.php

BlockPluginHasSettingsTrayFormAccessCheck.php in core/modules/settings_tray/src/Access/BlockPluginHasSettingsTrayFormAccessCheck.php

BlockPluginHasSettingsTrayFormAccessCheckTest.php in core/modules/settings_tray/tests/src/Unit/Access/BlockPluginHasSettingsTrayFormAccessCheckTest.php

BlockRepositoryTest.php in core/modules/block/tests/src/Unit/BlockRepositoryTest.php

Contains \Drupal\Tests\block\Unit\BlockRepositoryTest.

BlockSettings.php in core/modules/block/src/Plugin/migrate/process/BlockSettings.php

BlockStorageUnitTest.php in core/modules/block/tests/src/Kernel/BlockStorageUnitTest.php

block_test.module in core/modules/block/tests/modules/block_test/block_test.module

Provide test blocks.

Broken.php in core/lib/Drupal/Core/Block/Plugin/Block/Broken.php

color.module in core/modules/color/color.module

Allows users to change the color scheme of themes.

ConfigureBlockFormBase.php in core/modules/layout_builder/src/Form/ConfigureBlockFormBase.php

contextual_test.module in core/modules/contextual/tests/modules/contextual_test/contextual_test.module

Provides test contextual link on blocks.

help.module in core/modules/help/help.module

Manages displaying online help.

menu_ui.module in core/modules/menu_ui/menu_ui.module

Allows administrators to customize the site's navigation menus.

SectionComponentTest.php in core/modules/layout_builder/tests/src/Unit/SectionComponentTest.php

SectionRenderTest.php in core/modules/layout_builder/tests/src/Unit/SectionRenderTest.php

system.module in core/modules/system/system.module

Configuration system that lets administrators modify the workings of the site.