The "Complications" item is added in the makeLimitReport()
function, with contents generated via getAllFlags()
. The information that getAllFlags()
returns is in turn defined via setFlag()
(though that's tagged as deprecated in favor of setOutputFlag()
; your guess is as good as mine for why the deprecated function would be used here). I don't know what calls setFlag()
, or with what values, or when any of this was added. Poking around some on Phabricator, this field has existed since at least November 2019 (per this comment), but I couldn't find any mentions of it beyond incidental inclusions in copy-pasted NewPP output.
EDIT: looking around on en.WP, I also found this mention in August 2019, which pushes back its introduction a bit more.
I know this isn't much, but it at least narrows down some when it could've been added, and maybe it'll help you out a bit.