Is it on-topic if the programming question is specific to a retro computer or CPU? Examples: Apple II, or Zilog Z80.
What about an obsolete, platform-independent OS or programming language? Examples: GEM, Pascal, or BASIC.
As already mentioned in comments, there is considerable overlap between this question and Are x86 programming questions on-topic? from a few months ago. Indeed that question attracted comments saying that it shouldn't apply to just the x86 range.
I would say that the answer is the same as the consensus reached there. Any question that seeks to understand retrocomputers and their processors through programming is on-topic. I would include in that the tricks used to get code into the small amounts of memory available. Tricks rendered largely obsolete by cheap memory.
It needs to be judged on a case to case base as it's not a strict black and white issue but a continuum.
Just being no longer sold or in less use doesn't make something RC. The 8086 discusion Chenmunka mentions is a good exxample - and it even extends to Z80, as ofsprings like the eZ80 are still used for new development.
The question about a C64 BASIC problem is a another great example. The OP didn't use any classic hardware or tools and the problem was not even related to any, but a basic BASIC error. Still, noone flagged it as off-topic or moved it over to Stackoverflow. So, would this make a missing comma in a printf() on topic, just because the poster is using ORCA/C for the Apple II?
I do not belive that certain keywords (like
C64) trigger automatic on-topicness.
Neither will some still in use technology create a general exclusion.