I think that, generally, design review type questions are on-topic. However, the problem is how broad they are. My concern for this type of question is that most of them may be more suited to a discussion environment. I do think that there are good design review questions, but they need to be clear and specific and not soliciting general feedback. Specifically, in the question you linked to, this section has me most concerned over its viability:
I'm looking for a general review of my OO design, use of UML, and use
of C++ features. I especially just want to know if I'm doing anything
Any sort of feedback is greatly appreciated. Thanks for reading this.
I would expect a good design review question to focus on a particular aspect of the design (or the representation of the design, which would also be valid). I'm not sure this particular question does that well. But it's not off-topic, just too broad or unclear.
Borrowing from gnat's answer (which borrows from the guidelines used at Code Review):
Does your question contain design? Design should be embedded in the body of the post and not linked to. Design notation should be explained to ensure that it is well understood, especially if non-standard notations are used. Design should contain text, images and diagrams, and perhaps some examples of code if necessary.
Do you want the design to be good design, (i.e. not design-golfing, obfuscation, or similar)? We are practical people on Software Engineering, and many of us are professional software developers. We answer questions in the context of building production-ready systems that are understandable, maintainable, and meet other functional and quality requirements.
To the best of your knowledge, does the design work? Your design should at least be functional. You may be seeking input on improving your design, or perhaps correcting issues that are preventing you from achieving your quality requirements (such as performance or testability). However, your design should work and achieve the functional requirements of what you are building.
Have you identified specific areas of concern in your design? Simply dropping in a functional design with details isn't sufficient. What aspects of your design need improving? Are you having trouble testing your design (at any level of testing)? Is your design not performing as well as it should or needs to (and in what ways is it under performing)? Are you having trouble scaling your design? You should come with a small number of specific problems that you are trying to get resolved.
If you answered yes to all the above questions, your question is on-topic for design review at Software Engineering.