Are questions on simple class hierarchies, like this recent one, on topic? I think they are perfectly on topic here, but also that they might find a much better home at Code Review SE. As @YamMarcovic notes in a comment to the question, a class diagram is essentially code. Initially I agreed, but now I'm a little bit undecided.
Code Review SE is still in it's early days, and unfortunately they still haven't decided if class hierarchies are on topic there, as the relevant meta discussion is still open. Their faq specifically disqualifies questions on "Higher-level architecture and design of software systems", and a meta question on those points to Programmers as the appropriate site. But can we really classify a simple class hierarchy question as a "Higher-level architecture and design of software systems" question?
If we decide that they are off topic here, should we care that they might not find a better home at Code Review?