I am thinking about the machines envisaged and built by Charles Babbage and contemporaries. These are general-purpose computing devices which are not current.
Are questions about these systems on-topic?
I would say yes they are.
There are a number of efforts in museums around the world to (re)create the likes of ENIAC, Colossus, the Babbage Difference Engine and others. Indeed we already have a question about ENIAC.
Also, museums are working on emulators so as to educate people as to the workings of these old machines. This too should be on-topic in my opinion.
As to how far back you can go and still call a machine on-topic, that is open to debate. The Antikythera mechanism or the Stonehenge Aubrey Holes are likely to be considered off-topic.