13 files declare their use of SubformState in Drupal 9

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

FieldLayoutEntityDisplayFormTrait.php in core/modules/field_layout/src/Form/FieldLayoutEntityDisplayFormTrait.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