In summary historical population projections have been pretty accurate but not in the way you expect.
*One thought is that because current date may become a historical inflection point similar to the first half of the 20th century, current date demographic statistics may be too important to put much stock in. Sobering.
Unfortunately pre-1950 demographic prediction is a little harder to find. I wonder why*
Anyway a reliable source is found here: https://ourworldindata.org/population-projections
In summary historical population projections have been pretty accurate but not in the way you expect.
*One thought is that because current date may become a historical inflection point similar to the first half of the 20th century, current date demographic statistics may be too important to put much stock in. Sobering.