It is obsolete since 2006 (https://invent.kde.org/unmaintained/kdelibs/-/commit/d54c335d6e8ab2127d74fcbfea52dd0e9a42262a).
There is a TODO KF6 on removing the relevant code from KCModuleLoader (https://invent.kde.org/frameworks/kcmutils/-/blob/master/src/kcmoduleloader.cpp#L115)
It is still used by some PIM apps (knotes, korganizer, basket, kleopatra, kmail).
https://invent.kde.org/pim/knotes/-/commit/9ada31cbbc4ddb26c60ed9c1486ded62d866202f
https://invent.kde.org/pim/knotes/-/commit/f269d880ef732263bc2949e5b17bcdd7f17315f9
https://invent.kde.org/pim/korganizer/-/commit/5fd15ecce428c9c6239012bb9be7181cd13be01c
https://invent.kde.org/pim/kmail/-/commit/d938985b1d271f4a8ce34d5cb9f5eb231ab2526a
https://invent.kde.org/utilities/basket/-/commit/847c24a3dac9a75d2709670a5574fedd59178ed5
https://invent.kde.org/pim/kleopatra/-/merge_requests/7
https://invent.kde.org/utilities/kdf/-/commit/ae6d5f487b59643696283a64c055f756022b0a34
kleopatra
The remaining create_ functions are not exported or used to create any KCM.
basket
MR is already merged
kdf
Done