Any fertility rate below 2.1 isn't reduced population growth. It is the literal, factual opposite of population growth... it is population shrinkage/decline/whatever.
These countries we discuss all have population decline. This is masked by multiple factors including immigration and increased longevity.
This world doesn't have the 500 cultures/civilizations that it had circa 3000 BC. It only has the one culture, it's global, dominant, and ubiquitous. It is the "less natal" culture, it is the only culture. It assimilates any subculture that dares raise its head. When it dies, there won't be another to fill the niche.
This is why, for instance, when immigrants from high-fertility countries (all in central Africa) come here, our culture assimilates them and they're back to low fertility within 2 generations. Not only that, but our culture slowly creeps inward towards the epicenter of that high fertility, damping it down even at its source. The Chinese belt-and-road initiative is likely to murder it entirely within our own lifetime.
This is something different, never seen before. Saying "oh gosh shucks, it happened back in eleventy-twenty-fourven and we survived fine then!" is dismissal, not contemplation. Never before in human history has actual fertility sunk this low (or even really sunk). People didn't ever stop fucking, and they didn't have birth control, or abortion, or a dozen other factors. When population declined historically, it was due to war, disease, and famine. But because fertility remained high, just as soon as those pressures relented, population recovered. When fertility goes below replacement, population can never recover, it can only continue to shrink. When it goes below replacement, every girl growing up learns from the adults around her that low fertility is "normal", and she's not going to grow up and decide to have 8 herself (or even 2). She'll have as many as she saw those adults around her having, or maybe fewer (it acts as a ceiling, but not a floor).
It's not “developed countries have reduced population growth”.
It's: ALL countries have reduced population growth in recent decades compared to their historical baselines. Including in "non-developed" countries, like in Africa.
Many (most) countries have also dropped to sub fertility rates or close above.
On top of cultural and other reasons, there are also objective fertility issues with sperm counts and others emerging (likely due to modern food, climate crisis, microplastics, or some such).
Combine that with looming issues emerging from population shrinkrage causing economic decline, pension collapse and things like that, and then add environmental issues and resource wars into the mix.
It's no consolation if some pockets of humanity here and there carry on the torch, if humanity shrinks down to irrelevance.
