Population reduction is not envisaged as a practical (never mind humane) solution to CO2 emission reduction as far as I can tell.
I stumbled however on a paper discussing the estimated effects of such a past event, i.e. Europeans colonizing the Americas, Koch et al. (2019):
Estimates European arrival in 1492 lead to 56 million deaths by 1600. [...]
European epidemics removed 90% (IQR 87–92%) of the indigenous population over the next century. This resulted in secondary succession of 55.8 Mha (IQR 39.0–78.4 Mha) of abandoned land, sequestering 7.4 Pg C (IQR 4.9–10.8 Pg C), equivalent to a decline in atmospheric CO2 of 3.5 ppm (IQR 2.3–5.1 ppm CO2). Accounting for carbon cycle feedbacks plus LUC outside the Americas gives a total 5 ppm CO2 additional uptake into the land surface in the 1500s compared to the 1400s, 47–67% of the atmospheric CO2 decline.
Are there any other depopulation events for which such CO2-reduction estimates were derived?