It seems that questions about electronic calculators are on topic
Erm. Yes but. The answers for the cited question seams to go for programmable calculators and calculator like computers.
And that's where I'd draw the general line as well. Is it programmable - in the most generic sense(*1) - then I'd definitively see it as on topic. If not, a question needs to provide a real good reason to tie it in here. Like asking about construction details or algorithms used. In fact, there are quite nifty solutions mechanical devices have used which may be of great interest way past it's genuine implementation.
This is important to keep it focused on computing - not mechanics or mathematics, or programming.
Bottom line: If not programmable, it needs to be reasonable tied to RC.SE.
After all, a good question is not about a specific technology, but a relevant issue.
I'd like to extend this question: Are questions about following things on topic?
Mechanical Calculators
Sure - see above.
Slide-rules
Not really. While quite cool - and falling under the aspect of analogue computing - they are merely a lookup table for precalculated values. Without any additional mechanic, a slide rule doesn't compute anything. Thus they are in the general field of mathematics, not computing.
Having said that, I still could see question about historical development being on topic
- but these may be rather rare cases.
Mechanical Computers
Of course. The major point is being a computer. For sure if they are programmable, and usually as well with fixed function. And despite having a heavy focus on micros, RC.SE members love to share knowledge about older technologies.
Analog Computers (using an continuous encoding, e.g. the differential analyser)
No matter what build, they all fall under the same rule as mechanical or whatsoever based computers.
Punched Cards
Not really a computer - but yes, media in general is as well on-topic and punch cards (and tapes) have been targeted many times.
I'd love to see more exotic constructions, as they make me learn new ideas.
*1 - As in any automated processing, considered in the widest sense possible. There is no need for sophisticated features or being Turing complete at all.