15 files declare their use of WorkflowInterface in Drupal 8

ContentModeration.php in core/modules/content_moderation/src/Plugin/WorkflowType/ContentModeration.php

ContentModerationConfigureEntityTypesForm.php in core/modules/content_moderation/src/Form/ContentModerationConfigureEntityTypesForm.php

ContentModerationTestTrait.php in core/modules/content_moderation/tests/src/Traits/ContentModerationTestTrait.php

content_moderation.module in core/modules/content_moderation/content_moderation.module

Contains content_moderation.module.

ModerationInformationTest.php in core/modules/content_moderation/tests/src/Unit/ModerationInformationTest.php

StateTransitionValidation.php in core/modules/content_moderation/src/StateTransitionValidation.php

StateTransitionValidationInterface.php in core/modules/content_moderation/src/StateTransitionValidationInterface.php

Workflow.php in core/modules/workflows/src/Entity/Workflow.php

WorkflowCustomAccessType.php in core/modules/workflows/tests/modules/workflow_type_test/src/Plugin/WorkflowType/WorkflowCustomAccessType.php

WorkflowStateDeleteForm.php in core/modules/workflows/src/Form/WorkflowStateDeleteForm.php

WorkflowStateTransitionOperationsAccessCheckTest.php in core/modules/workflows/tests/src/Unit/WorkflowStateTransitionOperationsAccessCheckTest.php

WorkflowTransitionDeleteForm.php in core/modules/workflows/src/Form/WorkflowTransitionDeleteForm.php

WorkflowTypeBase.php in core/modules/workflows/src/Plugin/WorkflowTypeBase.php

workflow_type_test.module in core/modules/workflows/tests/modules/workflow_type_test/workflow_type_test.module

Module file for workflow_type_test.

WorkspacesContentModerationStateTest.php in core/modules/content_moderation/tests/src/Kernel/WorkspacesContentModerationStateTest.php