/etc/xdg/purpose.categories is in libkf5purpose5 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 | kdevplatform.plugins.phabricator purpose (phabricator plugin)
kdevplatform.plugins.reviewboard purpose (reviewboard plugin)
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