Can I put my NSF fellowship on reserve in order to investigate leaving academia?
There do not appear to be any reservations on the purpose for requesting a deferral in the linked guide. You need to tell them in advance of filing the "Declaration To Utilize, Reserve Or Terminate A Subsequent Year Of A 3-Year Graduate Fellowship Award" and get their approval, but I don't think that there are many reasons for them to deny it. You might want to have something lined up like an internship, other job, or visit to a foreign university rather than just saying that you're taking a year away from studying. Though, I wouldn't be surprised if the latter happens, too.

Edited to add: You need to start talking to your supervisor and eventually the NSF Program Officer in charge of your fellowship.