Yeah, interestingly scientific racism is on the list of banned topics for that reason, because it’s straightforwardly nothing but a way to smuggle in some white supremacy tarted up in the dispassionate academic language of data and studies and charts and statistics. It’s always thoroughly discredited but “let’s look at the data” serves as nothing but a sufficient wedge to get people’s guard down and entertain white supremacy.
“The birth rate” is I think a little different. I think for the most part (or at least for most Dopers’ part) they’re not consciously trying to soft-sell white supremacy, but they are unwittingly laundering some racially motivated messages from elsewhere. The putative effect is the same, it ends up creating pressure against reproductive freedom and immigration in the same stroke.
Of course the solution to falling birthrates is to get women back to being baby factories, of course this undercuts the idea that we actually need and benefit from immigrant labor, if we can get creative about policies to help us reproduce our own workers (weirdly nobody seems too worried about whether too many Black babies are being produced, and there are some disturbing implications about why a ballooning Black population is seen as a manageable situation).
The hand-wringing over “the birth rate” isn’t quite the repeat offender as scientific racism (at least not yet) but it very much is scientific xenophobia and misogyny, whether the participants realize it or not, and it should be on the shortlist for inclusion in the “tired topics” list next to scientific racism.