kelibs4support still sports the KDEUIMacros.cmale file, which defines a macro KDE4_ADD_WIDGET_FILES, which uses KF5DESIGNERPLUGIN_GENERATOR_EXECUTABLE, so has a runtime dependency on kdesignerplugin with the development files.
In that case it does not make that much sense to also remove it as build time dependency? Potentially resulting in packagers changing too much things, and forgetting about the run time dependency?

Thus discarding for now.