It was €4 for unlimited use for a day.
I'm not sure making it free is the right choice. I think it will lead to increases in vandalism and other crime. But, Luxembourg isn't renowned crime rate, so it may be ok. But If they tried this in Brussels or London, then it would lead to the crime issues above.
In Amsterdam GVB have to meet certain requirements for fare enforcement. Not because of any worries about revenue, but because fare evasion is an indicator of other crime on the network.
J