For a lot of countries, it did go away.

I think if you look closely it was just hidden behind various structures, like e.g. hiring seasonal workers from abroad, housing them in your own (substandard) buildings, and cutting their wages by the amount of rent that you charge so their take-home is well below minimum wage. But they tolerate it because it's still better pay than they can get in their home country.

It may have been made illegal, but it hasn’t gone away.

While slavery exists in various forms (wage slavery, prison labor, etc.) it is still important to make the distinction between those and the extreme form of chattel slavery that existed from the 16th to 19th century. People, and all their descendants, being literal property, able to be tortured, raped, and murdered with the same legal status as a couch tossed in a dumpster, is not the same as modern versions of slavery.

My impression from school was that in the well-known slavery societies, slaves or serfs outnumbered free men.

Maybe the number of sailors in deplorable conditions is equal to number of prisoners, worldwide. 0.2% of world population?