This Meta discussion is prompted by this question which I had flagged as off-topic, but which was declined. It has come back onto the active questions page, so I'm bringing it here.
I couldn't find any definitive record in Meta about whether questions which ask about the scientific plausibility of something should be considered off-topic or not. I think they definitely should be. Some of these questions are Truthy, some are just off-topic.
(Note that this is different from asking how a group (such as creationists) justify claims that something is scientifically plausible.)
FWIW, the Sci-Fi stack exchange also says these questions are off-topic. And some of them could be migrated to Sceptics.