6 files declare their use of FlowControllerSimple in CMS Content Sync 2.1.x

Embed.php in src/Controller/Embed.php

Flow.php in src/Entity/Flow.php

MigratePull.php in modules/cms_content_sync_migrate_acquia_content_hub/src/Form/MigratePull.php

MigratePush.php in modules/cms_content_sync_migrate_acquia_content_hub/src/Form/MigratePush.php

SimpleFlowSetupHelper.php in src/Helper/SimpleFlowSetupHelper.php

SyncCoreEntityItemResource.php in src/Plugin/rest/resource/SyncCoreEntityItemResource.php