Any recourse for testing on material not mentioned in the class syllabus? [closed]

In one particular university class, many people stopped attending after about half the semester was over as the professor was "confusing" as many people phrased it and opted to instead study from the textbook.

In retrospect, I hope you see the flaw in this approach. If your instructor is confusing about something, you go speak to them, in person, to get to the bottom of the things that are confusing to you. Your instructors are there to teach you this stuff because a majority of students are not adept enough to do it on their own.

The course syllabus outlines the chapters in the textbook that would be subject to testing.

Well, in my syllabi I say something like "this outline is tentative, and subject to change." Assuming your instructor did not have this disclaimer in the syllabus: too bad. A syllabus isn't a legal contract. You'll know better next time.

It would be "nice" if instructors could always think of every little thing to put in a syllabus so that whiny students who don't bother showing up, etc. wouldn't come and complain about how a lack of effort on their part was somehow someone else's fault but their own.

Is there any recourse now in terms of marks or having the question removed in your opinion? Furthermore, could you provide on brief response as to whether you think the prof was justified in doing this?

I do think your instructor was justified. However, I'd also like to point out that you have options, if you really do think you were not given a fair exam. For example, you could go and talk to the course instructor about it (you know, like you should have when the course got to be "confusing").

I hope more than anything that you learn a valuable lesson here: when things don't go your way, don't look around to find someone else to blame — take ownership of your actions.