The claim overstates the legal reality. Slavery is broadly prohibited under international law, but that is not the same as every country’s own law clearly making slavery illegal. Credible legal research shows many states still lack a specific slavery offense, and some legal systems retain exceptions such as penal slavery, so the absolute “every country” claim does not hold.
Caveats
International prohibition and domestic illegality are not the same; the claim blurs that distinction.
A country’s failure to create a specific slavery crime does not always mean slavery is affirmatively legal, so broad country counts require careful interpretation.
Recent or sensational claims about a country newly legalizing slavery were not solidly corroborated in the cited high-authority sources and should not be treated as decisive.