10 years ago at the time of writing, this question was asked:
Superuser vs Unix/Linux vs Ubuntu
regarding the distinction and overlap of scope between the three StackExchange website.
The accepted answer emphasized how things were in flux:
Until those sites come out of Beta, the option is really open at this point. Right now we still welcome Ubuntu/Linux questions, and since the sites may get merged before they are out of beta, SU will still be the most visible point since it is actively indexed and open.
Well, the sites have graduated and enough time has passed to pose the question again.
So, in 2020, what is an appropriate rule of thumb...
- for choosing where to ask among the three websites (for an issue which might fit on at least two of them, considered separately)?
- for migrating new questions between the websites?
Note: I'm not asking about migrating questions a good while after they've been asked (which is the subject of this question).