It is not just singapore . It is common in many parts of the world like middle east.

Economic Slavery has always been there and is still there. There are plenty of jobs which people don't want to do unless they are poor. Either countries kept parts of the population always poor or imported from poor countries. Like mexicans in america. Exploiting the working class is a feature of modern variants of capitalism.

Legal slavery is the only thing that got abolished.

P.S. on a seperate note and no means a defense of this, the salary is slightly misleading as it usually includes stay and food.