This was introduced in March 2008 with r32228
<https://www.mediawiki.org/wiki/Special:Code/MediaWiki/32228> (ad178edb80) for use
in the CategoryPager of Special:Categories, but almost immediately removed again from
there, on the same day, in r32259
<https://www.mediawiki.org/wiki/Special:Code/MediaWiki/32259> (bdf9431795) because
it wasn't actually supported.
There has been no other use of it to my knowledge, and has remained undocumented and
without tests.
I'm proposing removal without deprecation (per Stable interface policy#Removal
<https://www.mediawiki.org/wiki/Stable_interface_policy#Removal>). Note that if you
did use it outside publicly hosted/indexed repos, you don't risk visible breakage and
require no migration. Unused method overrides in subclasses are silently ignored.
https://gerrit.wikimedia.org/r/c/mediawiki/core/+/850612/
<https://gerrit.wikimedia.org/r/c/mediawiki/core/+/850612/1>
-- Timo