Manual:$wgArticle

Details
The Article object encapsulates access to the "page" table of the database. The object represents an article, and maintains state such as text (in Wikitext format), flags, etc.

The CategoryPage and ImagePage objects are child objects of the Article object, and are used specifically for Category pages and Image pages.

Replacement
Use the Context object to get what you need. Depending on what you need you may have to instantiate a WikiPage or call an Action class method passing the Title from getTitle