Neither the UN nor any nation formally condemned the US regarding its 13th Amendment’s exception clause. The important thing to recognize here is that what constitutes “slavery” can vary in scope depending on how you define it. While you could make a legitimate argument that the 13th Amendment’s exception provision essentially allows slavery to continue, many nations wouldn’t necessarily consider penal labor to constitute slavery. For example, the UN’s Nelson Mandela Rules, which are guidelines for humane treatment of prisoners, note that prisoners conducting penal labor deserve decent working conditions and a fair wage. While you could make the argument that the American prison labor system does not provide for fair wages, this would be a prisoner’s rights issue, not a slavery issue. The use of penal labor is not in itself a problem.

Further, the UN’s own page discussing the International Day for the Abolition of slavery discusses contemporary form of slavery to include forced labor, but makes no reference to penal labor as being slavery.

For further details re: Mandela Rules. https://www.un.org/en/events/mandeladay/mandela_rules.shtml

For further details on UN’s International Day for Abolition of slavery, with UN’s discussion on recognizing modern forms of forced labor as slavery. https://www.un.org/en/observances/slavery-abolition-day