Scarcity is not a requirement.
I disagree. If there is an unlimited supply of something available for free, it is not going to make a good currency.
Disagree all you want, but scarcity isn't a requirement, only a limited supply. They are not the same thing. I should say that scarcity is important in the sense that it affects the value of things, but only as the other side of the equation to demand; however scarcity alone doesn't create that demand. And if something is too scarce for the common person to have any practical experience with it, such as platinum before 1850, then it ends up being regarded as worthless metal and made into cannons for ships built to carry gold.