So we used to be able to make threads for our fictions, which served as a mighty useful place to put extraneous information that didn't necessarily come up in the story, or to organize it in a way helpful for the readers. However it looks like such threads aren't allowed in the general forums, has it been moved elsewhere, or is there something else which serves a similar function?

There was never such an option. All fictions used to be threads, which was removed a long time ago. You can use chapters to serve the same purpose, as they can be reordered at will.