There is now a FAQ item (it's pretty new)
Why was my feature/bug declined or closed without explanation?
Meta Stack Overflow has been around for a while, and as a result, many issues have been considered in the past. If your feature request has been marked as [status-bydesign] or [status-declined] without a comment, it is likely because there is another question that approaches the same topic and a conclusion has been reached. Perhaps you could try searching for your idea with different search terms, hopefully that will find the original discussion.