/etc/xdg/akonadi.categories is in akonadi-server 4:15.12.3-0ubuntu6.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | log_akonadiagentserver akonadi (akonadi agent server)
log_akonadiserver akonadi (akonadi server)
|