Since the new World Population Prospects were now leaked a few days before the official release date (July 11), I will collect my observations in this thread.
The post-2015 fertility collapse has now also partly been incorporated into the projections. World population is projected to reach 9.687B in 2050, to peak in 2086 at 10.43B and to reach 10.36B in 2100. 3 years earlier, it was like this: 2050: 9.736B, 2100: 10.87B and no peak.
Subsaharan Africa was slashed from 2.118B to 2.094B in 2050 and from 3.775B to 3.435B in 2100. This revision alone explains most of the downward correction in the world population projection.
Some other notable revisions (pop in 2100, projected 2019 -> projected 2022)
Answers depend on what is really meant here: Why fertility is generally low in the developed world, or why fertility has suddenly declined further in many developed countries in the last 10 or so years.
The answers to the first question are already well known anyway, so here are some possible explanations for the recent decline (in no particular order, just as they came up in my mind).
1. Structural changes from the 2009 crisis that makes it hard for young people to have good careers (particularly in Southern Europe, but in the Anglosphere as well).
Destatis released new figures on Covid birth trends in Germany from Jan-May 2021 - Births increased 1.4% to Jan-May 2020 (they compared final 2020 to prel. 2021 figures, so it's a different result compared to my birth table, where it's +3.9%).
Share of extramarital births went down from 33% to 32%. There were no changes in the distribution by birth order (46% firstborns, 36% second-borns and 18% third-etc.-borns).
The fact my followers are obviously interested most in is the distribution by citizenship: Share of births to German citizens up from 75% to 77%. Foreign share thus went down from 25% to 23%.