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 This has been superseded in later versions by the $wgNavigationLinks setting and MediaWiki:Sidebar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.
Usage search: https://codesearch.wmflabs.org/search/?q=SiteSupportPage&i=nope&files=&repos=
=== Actionables
[ ] Remove from `DefaultSettings.php`
[ ] Remove from rOMWC `InitialiseSettings.php`
[ ] Mark as removed on [[ https://www.mediawiki.org/wiki/Manual:$wgSiteSupportPage | it's documentation page ]]