This configuration global is unused in MediaWiki, and does not appear to have any usages in any extensions or skins.
The documentation page states that
This has been superseded in later versions by the $wgNavigationLinks setting and MediaWiki:Sidebar
but it was never deprecated. Note that $wgNavigationLinks has been removed in MediaWiki 1.5.0, which means that this setting is unused for over a decade.
Considering its lack of usage, I think we can forego a deprecation period as there are no known usages.