The requirements (i.e., Guidelines) are only in this document:
Web Content Accessibility Guidelines (WCAG) 2.0
W3C Recommendation 11 December 2008
Guidelines are usually technology-neutral and abstract, so they don’t apply only to HTML, but to Web content in general. Therefore, requiring WAI-ARIA attributes would make no sense.
However, to comply with the Guidelines, you can (but don’t have to!) use the Techniques for WCAG 2.0. It might be the case that certain Techniques rely on WAI-ARIA attributes. But, again, this is not normative: there are typically several applicable Techniques for a Guideline, and probably most of the time you might use a solution that is not described in a Technique at all.
So when you have to follow a Guideline, and a corresponding Technique describes a solution using WAI-ARIA, you don’t necessarily have to use it: Another Technique (not using WAI-ARIA) or an undocumented solution can possibly fulfil the Guideline, too. Of course, your specific restrictions might not allow for alternative solutions in all cases, so it can happen that a solution with WAI-ARIA is the only possible solution for you.