<p>I don't see why not.  That is a perfectly valid topic.  We have other, similar tags for methods used to infer causality from observational data, including:  </p>

<ul>
<li><a href="https://stats.stackexchange.com/questions/tagged/causality" class="post-tag" title="show questions tagged &#39;causality&#39;" rel="tag">causality</a> (387 threads)  </li>
<li><a href="https://stats.stackexchange.com/questions/tagged/granger-causality" class="post-tag" title="show questions tagged &#39;granger-causality&#39;" rel="tag">granger-causality</a> (137 threads)  </li>
<li><a href="https://stats.stackexchange.com/questions/tagged/instrumental-variables" class="post-tag" title="show questions tagged &#39;instrumental-variables&#39;" rel="tag">instrumental-variables</a> (292 threads)  </li>
<li><a href="https://stats.stackexchange.com/questions/tagged/propensity-scores" class="post-tag" title="show questions tagged &#39;propensity-scores&#39;" rel="tag">propensity-scores</a> (201 threads)  </li>
<li><a href="https://stats.stackexchange.com/questions/tagged/difference-in-difference" class="post-tag" title="show questions tagged &#39;difference-in-difference&#39;" rel="tag">difference-in-difference</a> (187 threads)  </li>
<li><a href="https://stats.stackexchange.com/questions/tagged/fixed-effects-model" class="post-tag" title="show questions tagged &#39;fixed-effects-model&#39;" rel="tag">fixed-effects-model</a> (511 threads)  </li>
<li>etc. </li>
</ul>

<p>As always, if you do create the tag, please add a good excerpt to guide usage, and preferably a good full wiki as well.  </p>

<hr>

<p>It seems we already have a <a href="https://stats.stackexchange.com/questions/tagged/discontinuity" class="post-tag" title="show questions tagged &#39;discontinuity&#39;" rel="tag">discontinuity</a> tag with 40 threads (ht, @amoeba).  Of those, 22 have both <code>[regression]</code> and <code>[discontinuity]</code> and certainly more than half are really about RD.  There are also 17 threads with <code>[regression]</code> and "discontinuity", but without the tag.  I think it would be better to have an explicit <code>[r-d]</code> tag specifically for RD.  The term / idea of <em>discontinuity</em> is somewhat generic.  It could cover RD, but could also be used for something like <a href="https://stats.stackexchange.com/questions/tagged/changepoint" class="post-tag" title="show questions tagged &#39;changepoint&#39;" rel="tag">changepoint</a> or something else (e.g., <a href="https://stats.stackexchange.com/questions/292784/is-there-a-way-to-correct-for-continuity-for-the-shapiro-wilk-test">Is there a way to correct for continuity for the Shapiro-Wilk test?</a>).  If we did create a new tag for RD, the question then is what to do w/ <code>[d]</code>.  My first thought would be to go through those 40 threads and properly retag them, leaving <code>[d]</code> only on those threads like the one just linked that are about other senses of discontinuity and that can't be given a more specific tag.  Then we could edit the wiki excerpt to provide better usage guidance.  </p>
