I absolutely concede that it's slavery. But to me, the thing that's bad about slavery is that its not voluntary. Slaves haven't historically chosen slavery. Frankly, any time you're forced to do things against your will, that's slavery. I would argue that conscription is slavery.
But if you volunteer, knowing at the time of your choice that you can't take it back, that seems different to me. It's like suicide. Should I criticise people who, fully aware of what they're doing, choose suicide? There may be reasons why I should, but it's not because of the self-harm, and the self-harm appears to be the only argument being made against voluntary golems. Like suicide, becoming a golem is a choice that effectively ends your life, that you can't undo, one that denies you future choices. So the morality around them should be similar, yes?
No as you are still conscious and might be condemning yourself to endless torture in form of performing acts you might not want to perform and yes you should criticize people who try or do commit suicide as they not only hurt themselves but also their relatives and friends, if they have some, and waste a whole lot of money the state invested in them. They should at least be considerate enough to take life insurance and let it appear like an accident.
Not to forget the whole "volunteering" is easily abused, especially in a caste system.
Again keeping the Anvil intact is absolutely the wisest decision but it's also the most despicable one in DAO.