edit: Apologies, looks like I was completely wrong since the source online I looked at incorrectly defined SK as economically undeveloped (I looked further and SK is in the G20, which would clearly make it a developed nation), and I was unaware of the Cold War era designation that commenters have pointed out to me.
I was always confused why SK would be considered a 3rd world country when my own impression from its industrial strength was that it should be a 1st world country, so I'm glad that I've been corrected.