There are even 21 questions on Arqade, tho' they go untagged there.
The current policy is outlined in our help center, which recognizes the overlap and states (emphasis mine):
When in doubt, ask there first. Questions where the Raspberry Pi is not a significant factor will likely be closed.
Real life close votes however do not be that stringent. It also points out what is on topic and what is off topic, which already covers some of your cases:
- Raspberry Pi hardware, including GPIO and other related electronics.
- Raspberry Pi peripherals.
- Software specific to the Raspberry Pi.
- Operating Systems built for the Raspberry Pi.
- Raspberry Pi orders or delivery.
- Asking for specific purchasing recommendations.
- Raspberry Pi accessories relating exclusively to appearance (such as cases).
- Questions directly related to Linux/Unix issues. Please use the Unix & Linux Exchange.
- General programming questions, e.g., "How do I implement a keyboard interrupt in python?". These will be better off on Programmers or Stack Overflow.
- Should the question be at least a little specific to the RPi? e.g. code must contain RPi specific features/libraries; the RPi must be doing something with the electronics; or the OS or features should be RPi specific
Yes, definitely. If phrased that way it's very ease to say. Yes, indeed, it should be specific to the Raspberry Pi. If it is a coding issue not related to the Pi it fits SO better. If it discusses general Linux issues that are not related the arm-build of the Pi or certain hacks to the distributions of the Pi it fits U&L better. (But there's more than one meta discussion to that.)
- Can the question be on-topic even if it's not specific to the RPi? e.g. is a general Linux question on-topic just because I'm running it on a RPi?
General Linux should be in U&L if the issue is not specific to the Pi but would happen on any other box too. I would argue that the user base is larger over there, so that better answers come up more quickly.
Well, probably there are. I think it would be best to discuss the rules as outlined in the help center. If amendments are to be made, the guidelines listed there should always reflect our current policy.
The existence of the listed questions at other branches of SE however is not only related to what is on/off topic here but also how things are handled over there, i.e. many of those questions were not migrated from RPi.SE to those sites but have been asked there, and - obviously - not considered off-topic.