Should questions about retro gaming consoles and related topic (their hardware, games and so on) be on-topic?
I tried to find an answer in the help, to no avail.
Yes they are.
They perfectly match the description in the current highest answer in the linked duplicate question and there are quite a few of them already posted on the main site like this one:
This issue was raised during the proposal process. While I don't generally consider Area 51 discussions binding, the top answer seems reasonable and well-regarded:
I Don't see any reason why it shouldn't be for certain types of questions. I'd argue that questions about particular games and how to beat them etc do not fit.
Questions about developing new software for old consoles, and repairing or modding them for modern AV would be a good fit.
I would allow these questions to see how they are handled… and see if a problem regarding their inclusion surfaces in actual practice.