That money is government property. It has been defaced. That’s why they can’t use real money in movies.
It's still perfectly legal tender. It is only a crime to deface currency for fraudulent purposes (like pass off a 5 as a 20). So it's still legal to write "Happy Birthday" on that $20 in the birthday card, and for stores to mark a bill with a marker to check for counterfeits, and for people to use those penny engraving machines, and for atheists to cross-out God.
It will be be destroyed within the same lifespan of all bills, about 18-22 months.
Personally, I think it's a stupid ideological shouting match from both sides.
That bill is a government IOU. But they have decreed it is legal tender, so...