Once upon a time, there was some text appended to the <title> like "- preview" or "- history" when you were doing an action to the page. This was implemented by the OutputPage::mPageTitleActionText member, assisted by its getter and setter, and OutputPage::setPageTitle().
November 2010, a cleanup commit (10ecfeb0) April 2009, commit e4d21170 disabled the functionality completely, presumably by mistake. It was reverted for other reasons, and recommitted as cadc951d.
Now that it did nothing, it only took until October 2011 for all callers to be removed, since after all it did nothing (2d045fa1, 9e230f30, e275ea28, ae45908c). So a feature seems to have been removed by mistake.
But not all is lost! As of July 2018, we were in the middle of advancing the state of MediaWiki development by adding lots more tests, and the issue was discovered. So we can restore the feature if we want, with tests so that it doesn't get removed again by mistake.