Legally or morally? I would say so morally. Legally is anyone's guess but you'd be crazy to take the advice of anyone other than a lawyer on that question.
I'm not really saying the old OSes were better. In many ways we have come a long long way. Yes the command lines in the old OSes were better. They were easier to learn and understand. They had features that seem to be missing in many shells today. Things like autocomplete for example. But mainly I think we need more new stuff. Stuff I don't know is missing. I don't want a return to the old but a look to what would be new.