29 files declare their use of WebformYaml in Webform 6.x

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

OverrideWebformVariant.php in src/Plugin/WebformVariant/OverrideWebformVariant.php

webform.install.update.inc in includes/webform.install.update.inc

Archived Webform update hooks.

Webform.php in src/Entity/Webform.php

webform.translation.inc in includes/webform.translation.inc

Webform module translation hooks.

WebformBlock.php in src/Plugin/Block/WebformBlock.php

WebformCliService.php in src/Commands/WebformCliService.php

WebformCodeMirror.php in src/Element/WebformCodeMirror.php

WebformDevelCommands.php in modules/webform_devel/src/Commands/WebformDevelCommands.php

WebformDevelEntityFormApiTestForm.php in modules/webform_devel/src/Form/WebformDevelEntityFormApiTestForm.php

WebformElementAttributes.php in src/Element/WebformElementAttributes.php

WebformElementComposite.php in src/Element/WebformElementComposite.php

WebformElementStates.php in src/Element/WebformElementStates.php

WebformEntityElementsForm.php in src/WebformEntityElementsForm.php

WebformEntityElementsValidator.php in src/WebformEntityElementsValidator.php

WebformEntityExportForm.php in src/WebformEntityExportForm.php

WebformEntityTest.php in tests/src/Kernel/Entity/WebformEntityTest.php

WebformImageSelectImages.php in modules/webform_image_select/src/Entity/WebformImageSelectImages.php

WebformOptions.php in src/Element/WebformOptions.php

WebformOptions.php in src/Entity/WebformOptions.php

WebformSettingsAdminTest.php in tests/src/Functional/Settings/WebformSettingsAdminTest.php

WebformSubmissionViewBuilder.php in src/WebformSubmissionViewBuilder.php

WebformTranslationConfigManager.php in src/WebformTranslationConfigManager.php

WebformTranslationManager.php in src/WebformTranslationManager.php

WebformTwigExtension.php in src/Twig/WebformTwigExtension.php

WebformUiElementFormBase.php in modules/webform_ui/src/Form/WebformUiElementFormBase.php

WebformYamlTest.php in tests/src/Unit/Utility/WebformYamlTest.php

webform_devel.module in modules/webform_devel/webform_devel.module

Provides development tools Webform module.

YamlWebformExporter.php in src/Plugin/WebformExporter/YamlWebformExporter.php