The idea behind the law is that the Gov’t does not want businesses requiring payment in foreign currency or gold coins, etc. (Accepting is OK, requiring is not). OTOH, this has not been a problem since the days of the “Continental Shinplaster” so there’s no case law, regs or penalties for the average citizen or even business doing so.
Deep inside Title 31 and FINcen bank regs, there is a regulatory section that prohibits Banks and similar Financial Institutions from doing so. There seems to be no criminal penalties attached, currently.
Again, the Treasury doesn’t really get into enforcement on this because it’s simply not a problem.