Aviation accidents in general are on topic and generate a lot of interest here, therefore investigating them is too.
I assume most people would agree that some things are unique to aviation accident investigations: interpreting FDR and CVR data; using radar tracks and ATC recordings; identifying flight attitude and direction based on impact patterns etc. There shouldn't be any problem with accepting questions about those areas.
On the other hand, there are also plenty of questions that aren't unique to aviation investigations: how can a pathology lab determine if a pilot/driver was intoxicated at the time of death; can an autopsy accurately determine cardiac failure (followed by loss of control) as a cause of death; how can an autopsy tell that a pilot/driver died in the post-crash fire and not because of the initial impact etc.
My opinion is that we should allow and even encourage questions about accident investigation - accident reports are very educational reading for pilots and others - and only close them if the question is generic and not specifically about aviation. I think it's better to encourage a topic and deal with the exceptions rather than discourage a topic to avoid dealing with the exceptions.