<p><strong>The short answer</strong>:  </p>

<p>Reality has a <a href="http://en.wikipedia.org/wiki/Status_quo_bias" rel="nofollow noreferrer">status quo bias</a>; the initiative seems to have simply withered away with time.  I doubt anyone (or the community as a whole) explicitly decided not to do it.  Either way, nothing along those lines came of that discussion, and there isn't an R.SE site listed in <a href="http://area51.stackexchange.com/">Area 51</a>.  </p>

<p><strong>Peripheral discussion</strong>:  </p>

<ul>
<li><p>I think you're right that there can be R questions that aren't a great fit for either SO or here.  One such question might be, 'is there an R package for <code>_____</code>?'  (Note this question comes up regularly and seems to be tolerated.) In addition there are questions that are off-topic on all SE sites by design (e.g., open ended discussion questions).  However, there are other sites on the internet--the r help listserv and forums, etc.  I don't believe that SE should try to expand to the point where it encompasses any question that could be formed in English.  </p></li>
<li><p>I agree with @whuber that having an R specific site <em>in addition to</em> CV and SO would likely lead to more confusion, not less.  </p></li>
<li><p>I personally have never felt any confusion about where a given question belonged unless the question itself was confusing (confusingly worded, for instance).  </p>

<p>For what it's worth, my criterion is simple: <em>what does the OP need explained to them?</em>  If someone were to ask 'how do you calculate the average in R?', that doesn't seem like my prototypical conception of programming, but they still need to know how to <em>use</em> R to get something done; an answer that explained what an average <em>is</em> would not help them.  </p>

<p>Here's another way to think about it:  R (in part because it's freeware, in part because it's common) is often used to <em>illustrate</em> a point in an answer on CV, but this is simply instrumental to the real answer.  If another language could have been used equally well, the question definitely belongs here; if not, it probably belongs on SO.  </p>

<p>Under any rubric there will be some threads that end up on one site that might better fit on the other; so be it.  For example there are questions on SO where someone asked why something worked like it did in R, but the problem isn't that they didn't understand R's syntax but that they don't understand the underlying statistical concepts.  There are some questions here (especially older questions that are too old to migrate) that really belong on SO.  </p>

<p>The argument is sometimes made that R programming questions with any statistical implications should stay here because a user would need to be statistically savvy to provide an answer that is sensitive to those (perhaps unstated) issues.  I don't buy this.  If you look at the <a href="https://stackoverflow.com/tags/r/topusers">list of SO users</a> who frequently answer R-taged questions, you will see that these people are quite statistically sophisticated.  Moreover, R is hardly used for anything other than data analyses of various sorts; almost anyone who is knowledgeable enough of R to answer SO questions will be familiar with statistics.  </p>

<p>For more information about whether a question should be migrated, you might want to read: <a href="https://stats.meta.stackexchange.com/questions/1514/">What standard should be applied in deciding whether a question requires sufficient 'statistical expertise' to be posted to CV?</a>  </p></li>
</ul>
