Not sure, but the birthrate has already fallen to 1.6 children per woman in the US, well below the required rate for a population of people to sustain itself(2.1). The US has hovered around the 2.1 replacement rate for a while but plunged below it since around the great recession. However, the US always has enjoyed a benefit other industrialized nations have not which allowed it to not have a shrinking population--robust immigration. As the US has currently done everything possible to make itself a destination that not only expels current visitors, makes becoming a citizen more difficult, and makes itself unattractive to potential immigrants, it is quickly falling into commonality with nations who have a demographic crisis(Japan, Italy, and many others) with the large aging post-war generation being unsupportable.

That's a long way to say, maybe the site isn't online anymore because they no longer have to advocate for something that has been realized/accomplished?