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. If you have a context object, say, you can use. Where you get this context object from depends on where your code is running. Many major classes extend, which means you can just do   to get the context. As of this writing that includes ApiBase, CategoryViewer, ChangesList, DerivativeContext, DifferenceEngine, HTMLForm, ImportReporter, IndexPager, OutputPage, RevisionListBase, and Skin. Some other classes support getContext which don't have ContextSource as a parent class, such as SpecialPage (So if you're writing a SpecialPage, you can often do  to get the context).

In many cases, you should also consider if you really want an Article object, or if you actually want a WikiPage object