Are questions asking for what the current research topics in a certain field allowed? Under what circumstance would they be? In case they are allowed, after how much time would it be allowed to ask the same question again, perhaps after a big discovery? Regarding this last point, while it's true that new answers could always be posted, it could also happen that the question stays buried unupdated.
EDIT: The reasons I think questions of this kind could be useful are:
- Undergraduate and Graduate students having to decide wich path to chose could use a clear explanation of some of the active research topics. Furthermore, they may not be used or don't have enough knowledge to understand what those topics are just by reading scientific journals. As Jon Custer noted in the comments, conferences may be an alternative, but it really depends on what you university actually offers and one could be interested in looking for something else anyway.
- While it's true that the answer changes with time, I think at any given time these questions could be answered. Of course, the answer is not unique, but so are answers to questions about references and those are allowed.