In chat, I asked about a possible undocumented core temperature sensor in a specific embedded processor. While the discussion itself was helpful, I was told that questions regarding undocumented features is off-topic on the main site because they would be opinion-based. Regardless of the topicality of the original question I wanted to ask, I would like to understand under what circumstances questions based on undocumented features or behavior are on-topic, if any.
My argument is that many well-known and commonly used features for microprocessors, FPGAs, CPLDs, etc. are undocumented despite being well-known and having established behavior. In fact, for some microprocessors, you cannot even run an emulator for them for any non-trivial program without supporting undocumented opcodes. So would questions regarding, say, undocumented opcodes, sensors, or JTAG ports be off-topic? Are they always off-topic, or only if the undocumented feature requires a significant amount of guesswork or is otherwise not well known?
From what I can see on the rest of meta, even questions regarding hardware which is entirely undocumented is on-topic despite every feature lacking documentation of any kind. I also see a number of well-received questions on the main site asking about undocumented features.