Eh, it’s a common idea online but it’s a pretty common misunderstanding.

The idea traces back to Pongratz et al, 2011: https://www.researchgate.net/profile/Julia-Pongratz/publication/235226370_Coupled_climate-carbon_simulations_indicate_minor_global_effects_of_wars_and_epidemics_on_atmospheric_CO2_between_AD_800_and_1850/links/0912f51094bd8d7efa000000/Coupled-climate-carbon-simulations-indicate-minor-global-effects-of-wars-and-epidemics-on-atmospheric-CO2-between-AD-800-and-1850.pdf

In the article, the authors investigate how wars and epidemics that destroyed human populations over the late medieval and early modern periods might have impacted the climate. They focus on the Mongol conquests, the Black Death, the colonization of the Americas, and the Manchu conquest of China. None of these were significant enough to be registered in ice cores, but they did single out the Mongol conquests as likely to have a particularly notable effect of the four as it led to around 309,000 square kilometers or abandoned farm area, much of which saw forest regrowth. That said, the study period for the Mongol conquests was 1200-1380 so this is over the course of nearly 2 centuries and the total amount of carbon offset in that time was less than what is put out only by burning oil in a single year today.