It's a bad idea. I'm not sure about Korea specifically, but a lot of developing nations have old/narrow pipes and they don't have the waste treatment facilities

It's a bad idea. I'm not sure about Korea specifically, but a lot of developing nations have old/narrow pipes and they don't have the waste treatment facilities to deal with toilet paper. They might not have the money to install advanced filtering or even activated charcoal. Often times the waste water is just diluted and dumped into oceans or lakes. I encounter the "toilet paper in the trash can" signs first in Moldova. The Nistru river has high levels of cholera, so it makes sense they simply do not have advanced treatment facilities.