/etc/xdg/knewstuff.categories is in libkf5newstuff-data 5.44.0-0ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | org.kde.knewstuff.core knewstuff (Core Lib)
org.kde.knewstuff knewstuff (knewstuff)
|