We didn't end slavery at all. There are more slaves now than ever.
https://www.nytimes.com/2023/08/01/business/economy/solar-xi... ("Solar Supply Chain Grows More Opaque Amid Human Rights Concerns / The global industry is cutting some ties to China, but its exposure to forced labor remains high and companies are less transparent, a new report found")
https://www.csis.org/analysis/dark-spot-solar-energy-industr... ("A Dark Spot for the Solar Energy Industry: Forced Labor in Xinjiang")
(Maybe there's some kind of evil Jevons Paradox for slavery, where the automation of human labor counterintuitively increases total slavery; i.e. the technologically-augmented effectiveness of slave labor increases the value of slaves).
If someone could show that paying a fair wage to workers would still leave solar compellingly cheap then it might incentivise some parts of the supply chain to clean up their act. That's "if" of course.
You can be certain that forced labor would be considered a "subsidy", although I don't recall ever having seen it mentioned in these filings, so my inference is that it's not a significant factor.
The last one I examined in any detail was https://www.federalregister.gov/documents/2023/07/11/2023-14..., which imposed a 10.33% countervailing duty on Jinko panels, a 14.27% countervailing duty on Risen panels, and a 12.61% countervailing duty on all other PRC panels. This was enough to ensure that under 1% of panels sold in the US were Chinese solar panels.
But we're talking about competitiveness with fossil fuels here, which are about 200% more expensive than solar power, not 14.27%.
In no way am I saying slavery is no longer a problem, one slave is too many. I chose not to go after the parent comment's claim that slavery has ended because that wasn't the important to the point I was raising.