economical theory
today's slaves are those people that offer themselves for cheap
Dog there is a world of difference between chattel "I have legal title to you and your descendents in perpetuity and can dispose of you as I wish" slavery and "I believe I am underpaid for the work I do".
Debt bondage existed and continues to exist. Most "sweatshops" are not good examples. Many pay worse than what a westerner would get, but provide better work hours, more reliability and similar or better income to subsistence farming, day labor or other viable alternatives.
, some places there may still be people that are forced into literal slavery (by force)
Mostly women, and mostly for sex. It is a serious issue. Some unfortunate people are tricked into coming to a country for work only to have their passport taken from them so that they can be forced to work in incredibly poor conditions with nil pay ad famously happened in Russia not long ago, but this is a much rarer event.