The CompletenessTest was my attempt at getting a basic code coverage report using run-time inspection instead of static instrumentation.
It was never fully developed, remained somewhat unstable, isn't used by Jenkins or otherwise enabled or encouraged, and its results are not publishable, either. (Only works locally as on the Special:JavaScriptTest HTML view).
The "export" feature for Special:JavaScriptTest introduced in 2014 for Karma and TestSwarm (ba50b3255) lacked support support for loading the CompletenessTest. And when the regular "skinned" mode was deprecated and, eventually, removed last year in 0f9e4ca0f it essentially hasn't been used anymore as far as I can see.
From a Git-wide search I see that various Wikibase repositories still have references to it, so I won't remove it just yet. But it'd be good to know for sure if and how it's being used there. There's no rush behind its removal, but if it's not being used, that I'd rather we remove it from core.
