I took the PL/SQL exam about a month ago and, to tell the truth, I don't remember any questions about decode at all. That doesn't mean there weren't any. As it happened I passed the exam easily, so there wasn't much incentive to try to remember specific questions for future review.
In general, you shouldn't spend large amounts of time trying to master the intricate details of specific commands. The exam is on a much higher level than that. There never seemed to be more than a question or two on any given subject area. To me it seemed that they weren't trying to trip up the exam takers on specific syntax. Instead they wanted us to demonstrate a broad range of knowledge in a variety of different subjects.