21 files declare their use of BlockPluginInterface

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.

Buggy or inaccurate documentation? Please file an issue. Need support? Need help programming? Connect with the Drupal community.