Only used by deprecated KInit:. Cmp. also class docs:"Represents an idle slave, waiting to be reused. Used by klauncher. Do not use outside KIO and klauncher! "

So does not need a IdleWorker replacement to match new terms. Instead can be just dropped

https://invent.kde.org/frameworks/kio/-/merge_requests/1039 proposes to mark the class with build wrappers.