There's still argument about this. The oft mentioned Dodge v. Ford Motor Company 1919 covers much the argument for and against. But it's clearly not straightforward.
My (IANAL) reading of it is that maximising shareholder value is probably the law, but it's practically unenforcible. Being practically unenforcible doesn't stop CEOs and boards from using it as a guiding principle.
Dodge v. Ford Motor Company was a Michigan State decision, so even if one thinks it means maximizing shareholder value is the law (it really didn't say that, broadly), it only applies in Michigan.
My (IANAL) reading of it is that maximising shareholder value is probably the law, but it's practically unenforcible. Being practically unenforcible doesn't stop CEOs and boards from using it as a guiding principle.
https://en.wikipedia.org/wiki/Dodge_v._Ford_Motor_Co.