I don't see anything wrong with the current challenge. I updated it to flexible IO a little while ago and added an actual specification. The entire body of the question is actually pretty modern. I even asked around if people thought it was clear and received only positive feedback. That doesn't mean its perfect but I would not say that the question body is old or outdated, and I am perfectly willing to take responsibility for any land all problems present in the writeup of the question.
I think that your complaint that it is "in general lacking answers" is simply false, as I mentioned in my comment it has 173 answers, which is a lot of answers. If this is a lack of answers I don't think anything can be done. Fibonacci is probably not interesting to the majority of users at this point and I don't see everyone rushing to add hundreds more answers anytime soon. If you want more answers you can post an answer yourself or bounty the question. Reposting the question is a surefire way to get less answers than there already are.
I also think that it is not "severely lacking" in specification (A little bit biased because I wrote the current specification, so if I thought it was incomplete I would have added to it). Any holes in the specification could be fixed with edits.
I don't think allowing for two types of solutions is a bad thing, flexible IO is in general good for challenges even though one is less conventional I think it adds to the challenge. The only reason I could see to not allow a method of output is to prevent abuse. I don't think there is anyway to abuse the current method and I think answers that use it are no less valid than answers that use a traditional method.
I don't understand your comment about try it online, so I wont touch that.
Overall I think the question is fine. Minor edits can be made, but I don't think we need to do anything drastic, like make a new question or overhaul the existing challenge.