12 files declare their use of WebformHandlerBase in Webform 6.x

ActionWebformHandler.php in src/Plugin/WebformHandler/ActionWebformHandler.php

BrokenWebformHandler.php in src/Plugin/WebformHandler/BrokenWebformHandler.php

DebugWebformHandler.php in src/Plugin/WebformHandler/DebugWebformHandler.php

EmailWebformHandler.php in src/Plugin/WebformHandler/EmailWebformHandler.php

ExampleWebformHandler.php in modules/webform_example_handler/src/Plugin/WebformHandler/ExampleWebformHandler.php

OptionsLimitWebformHandler.php in modules/webform_options_limit/src/Plugin/WebformHandler/OptionsLimitWebformHandler.php

RemotePostWebformHandler.php in src/Plugin/WebformHandler/RemotePostWebformHandler.php

SettingsWebformHandler.php in src/Plugin/WebformHandler/SettingsWebformHandler.php

TestEntityMappingWebformHandler.php in tests/modules/webform_test_handler/src/Plugin/WebformHandler/TestEntityMappingWebformHandler.php

TestPurgeHandler.php in tests/modules/webform_test_handler/src/Plugin/WebformHandler/TestPurgeHandler.php

TestWebformHandler.php in tests/modules/webform_test_handler/src/Plugin/WebformHandler/TestWebformHandler.php

TestWebformOffCanvasWidthHandler.php in tests/modules/webform_test_handler/src/Plugin/WebformHandler/TestWebformOffCanvasWidthHandler.php