There really isn't one. There just hasn't been a perceived need thus far. You could suggest a new site on Area 51, but we are the site for both applied and research-level statistics. Note that there is a lot of overlap amongst SE sites. A question that goes here can often also be on topic on Stack Overflow, Mathematics, Data Science, Artificial Intelligence, Operations Research, Quantitative Finance, Economics, among possibly others. Math Overflow does have an
[st.statistics] tag with 1,666 threads, so I guess people could post there, too.
I'm a bit skeptical, though. I don't think the proliferation of sites on SE is the best way to organize information. Also, people often post what they think are research-level questions that really aren't—an example might be the extremely poor use of our mathematical-statistics tag.