Id say “less” controversial.
Is making people in prisons perform the necessary custodial work to maintain it slavery? I dont think it is, but strictly speaking its enforced unwilling labor so by a dictionary definition it kinda is.
Now making them take jobs performing work for companies seeking to make a profit for less than minimum wage… yeah that smells a lot more like slavery.
What is and isn’t slavery is absolutely up for debate. You can have an argument over where you draw the line, but I’m pretty sure anyone remotely sane and moral would absolutely say that for-prisoner labour is slavery.
Which is probably why some countries specifically allow slavery for prisoners.