18 files declare their use of SubformState

BlockForm.php in core/modules/block/src/BlockForm.php
CKEditor5.php in core/modules/ckeditor5/src/Plugin/Editor/CKEditor5.php
ConfigureBlockFormBase.php in core/modules/layout_builder/src/Form/ConfigureBlockFormBase.php
ConfigureSectionForm.php in core/modules/layout_builder/src/Form/ConfigureSectionForm.php
editor.module in core/modules/editor/editor.module
Adds bindings for client-side "text editors" to text formats.
EmbeddedForm.php in core/modules/field_layout/tests/modules/field_layout_test/src/Form/EmbeddedForm.php
FieldConfigEditForm.php in core/modules/field_ui/src/Form/FieldConfigEditForm.php
FieldLayoutEntityDisplayFormTrait.php in core/modules/field_layout/src/Form/FieldLayoutEntityDisplayFormTrait.php
field_ui.module in core/modules/field_ui/field_ui.module
Allows administrators to attach custom fields to fieldable types.
FormController.php in core/modules/system/tests/modules/condition_test/src/FormController.php
ImageEffectFormBase.php in core/modules/image/src/Form/ImageEffectFormBase.php
MediaTypeForm.php in core/modules/media/src/MediaTypeForm.php
SubformStateTest.php in core/tests/Drupal/Tests/Core/Form/SubformStateTest.php
WorkflowEditForm.php in core/modules/workflows/src/Form/WorkflowEditForm.php
WorkflowStateAddForm.php in core/modules/workflows/src/Form/WorkflowStateAddForm.php
WorkflowStateEditForm.php in core/modules/workflows/src/Form/WorkflowStateEditForm.php
WorkflowTransitionAddForm.php in core/modules/workflows/src/Form/WorkflowTransitionAddForm.php
WorkflowTransitionEditForm.php in core/modules/workflows/src/Form/WorkflowTransitionEditForm.php

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