It can't possibly have been used anywhere, because it's not in the KConfig
API anywhere. The intended way to use this API was by providing a plugin
that would derive from KConfigBackend but the plugin loading code
in KConfigBackend::create has been disabled since before KF 5.0.
The reason I want to stop exporting this class is to be able to
optimize it (e.g. the QDateTime in it is completely unused but leads
to data races due to tzset)