Found only one external consumer of the SlaveConfig API in KDE repos currently, in office/tellico: https://invent.kde.org/office/tellico/-/blob/master/src/fetch/fetcher.cpp#L88
No idea if that could be solved in another way, and one could stop publishing the API?
https://invent.kde.org/frameworks/kio/-/merge_requests/1030 proposes to add KIO::WorkerConfig instead.