Terrorism is not Off topic
We create worlds, and those world may include anything. More importantly, we tend to create worlds that matter to us. And terrorism has stricken minds a lot in the past decades. So yes, terrorism is on topic, along with war, theft, rape, torture and any other horrid thing we know and hate.
Most things on this website are not new
This is in no way meant to disrespect the wonderful WB community. The thing is, most of WB's value (and most of SE) is not to create knowledge but to connect it. People on WB don't look for zero-day exploits or new bomb designs, they look for already existing ones. Now of course connecting information creates new information, but the addings of this community is that it would connect the terrorist plot to... the specificities of a world different from ours.
It would most likely be unusable anyway
Let's first ditch unusable and unrealistic answers (they exist), the big problem of a terrorist attack is not the overall plot, but the details of it. How secure are the communications between the terrorists, how to get the materials for the attack, how to go through the layers of security. From the idea of a thermonuclear device to its fabrication and then use, you'll need very very very very very talented engineers. If you have those, you're not gonna need some WB answers.
Putting in place that kind of moderation seems very problematic
Basically, "where to draw the line" seems very difficult. To me, considering the questions to be forbidden is threatening a lot of people with good intentions. How can you know if your question is going to help terrorists? Do you remove the question if it can, will or has created worthy terrorist information? How do you know what is terrorist-worthy? Why would you punish a question for its answers?
So you would punish the answers? That gives you even more moderating work to do, you are splitting discussion.
Mostly, refraining people from posting based on something that vague seems very bad.
Enough with that damn terrorist craze
Seriously. I know this is a very emotional topic, but that "we need to do something because we need to feel safe and we don't know what to do so we will do the first thing that seems to address the issue" is just creating an atmosphere of fear and distrust. Please don't bring that atmosphere to this site.