This was introduced in March 2008 with r32228 (ad178edb80) for use in
the CategoryPager of Special:Categories, but almost immediately
removed again from there, on the same day, in r32259 (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). 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.
-- Timo