You could argue that any kind of enforced imprisonment where someone is placed in chains and unable to leave of their own accord could be a kind of slavery, eve

You could argue that any kind of enforced imprisonment where someone is placed in chains and unable to leave of their own accord could be a kind of slavery, even if well treated. So in that sense the exception could make sense. But certainly it looks like it is time to amend that exception with further requirements for the humane treatment of prisoners.