Here goes another modern Robber Baron exploiting his employees and avoiding paying people what they're due… and avoiding the taxes that come with it. Pretty sure Musk's company-wide email setting an expectation of “long hours” is going to be an HR nightmare when people who know their rights start standing up for them.
Just because an employee is salaried or exempt doesn't mean companies don't owe overtime based on certain criteria.
This fluctuates state by state, and blue states especially have strict rules about this (like NY). Whether you work at Twitter or not, look up your state's rules and start fighting back. If your state hasn't started setting parameters around this, call your legislature. Enough is enough!