I'm not sure what you mean, but I don't think many countries still had legal slavery going into the 20th century, outside of the Arab peninsula.
In Korea it was officially abolished in 1894 (although it took a few decades to eradicate). In my own country, France, slavery was abolished in 1794 (in Europe) and in 1848 (in all colonies and oversea territories). I think it's the general timeline for western Europe, first half of the 19th century. So not exactly 20th or 21st century either.
Of course, slavery itself (especially related to prostitution) is still ongoing pretty much everywhere, but it's not legal. And then there's war: forced work (Germany and Japan during WWII), and arguably conscription could count as a form of slavery too...
I'm pretty sure I learned about it at school in France, although of course not each country individually. Not sure about your specific country and history curriculum.
It sounds like the authorities where you live should have a look at not just the history curriculum but the rest as well. Slavery is very much a contemporary issue and it's been around, globally, since times that predate written history.
I had no idea about Korean slavery that existed till the 20th century. Why aren't these parts included in history curriculum?