Broad questions are not in a good form to be answered for the reason you exposed, so they are indeed off-topic on any StackExchange site.
Generally speaking, generic guidelines about asking questions apply on the whole StackExchange network, and this site is no exception.
That being said, we still have to keep in mind the context and the nature of each specific site. Some subjects are more suitable for quickly determining whether a question is too broad or not, like for instance on StackOverflow: when talking about coding is much easier to identify precise questions and to understand which is the right answer (well, unless you program in Perl, but that's another matter).
On language sites, unless the question is trivial enough for being answered without any doubt, there's always room for different interpretations and a slightly broader discussion.
However, you are right, we shouldn't use this site a forum for discussing. From the help center
If your motivation for asking the question is “I would like to participate in a discussion about ______”, then you should not be asking here.
Specifically I just cast my first close vote on this question
Can Dante Alighieri be compared to Shakespeare as both fathers of their respective languages on the lexical level?
It was a suffered decision, but I after thinking about it for a while, I feel like the question is definitely too broad and primarily opinion-based.
As the OP said, such question was inspired by
which, while it may look on the same lines, it presents an important difference: the question is in the form There exist someone like X?, which is suitable for being answered in a succinct form Yes, Y is the equivalent of X, for the reasons A, B, C that support my claim.
It's still opinion-based, so probably not a great fit, but in the specific case opinions can be supported by facts and examples, so somehow it fits in the SE format.
On the other hand, the (very interesting) question martina posed, is proposing a broad comparison between the Dante and Shakespeare, which is very likely to lead to extended discussions and no direct answers, i.e. the kind of questions we would like to avoid here.