Not getting paid, happens.
Certainly there have been, and are, places with no “minimum wage” laws, per se.
And, people do work for no wage, pretty often in some circumstances, either as volunteers, company owners, or “on spec(ulation)” projects, for “a share of the profits”, or for tips, or for in-kind or service compensation, or to satisfy judgements, etc.
I’ve been offered “glory points. Lots of glory points,” and I’ve done work as one of several “tests” for approval for possible future at-will employment, for no direct compensation. (Game industry work, where I was interested enough in the projects to want to do that enough.)
It’s not slavery unless one must do the work, can’t resign, you’re considered the property of someone else, etc.