Where does the special-page's setup file (Randomrootpage.php) go if it is located in /include/specials by default? The default special-pages seem to be registered somewhere else than in LocalSettings, too.
The setup file goes away, core special pages get registered in includes/specialpage/SpecialPageFactory, autoload.php, etc. Aliases are somewhere in the languages/ directory, you'll have to look to find them.
The extension has been unconfigured from CI https://gerrit.wikimedia.org/r/#/c/264975/
I marked it read-only in Gerrit https://gerrit.wikimedia.org/r/#/admin/projects/mediawiki/extensions/RandomRootPage