My version of this is to just sterilize 99% of the population at random, and keep sterilizing people in the future such that the world population is capped at 100 million.
Good news: its happening in developed countries already. South Korea's birth rate in 2020 was 0.84 kids per woman. In order to keep population stable you need 2 births per woman. I think there is an imminent collapse in demographics that will be coming within the next 20-30 years.
Developing nations still have higher population growth but eventually itll all even out.