How should I cite an archived webpage?
I would consider doing both - the original (usually with a date accessed anyway these days - although I don't do much website citing), and a note that it is still available (as of your article) on the Wayback machine at the given address. 

Now, it is always (sadly) possible that Steam would consider requesting the Wayback folks to delete their listing in the future, but it provides a hint. 
Further, I'm surprised that Steam did not migrate the old site over to their new site, but that might have been a lot of work.