Michael Kjörling wrote:
> https://retrocomputing.stackexchange.com/
>
> Many of the questions there currently relate primarily to personal
> computers and consoles, but I see no reason why questions about e.g.
> historical mainframes would not be welcome.

I brought that up during the proposal stage, and I gather the response
was positive:

https://area51.meta.stackexchange.com/questions/22734/what-about-biggish-iron

Now there seems to be a handful of questions about minis and mainframes.
And, on topic!, Unix.