The FDA didn't ban it: it just cannot be marketed as a health supplement. Mostly because it was originally approved as a drug back in the 1960s. It would be like selling Tylenol as a vitamin. A dumb technicality that'll likely be reversed.
And you can still purchase it online:
https://www.bulksupplements.com/products/n-acetyl-l-cysteine...
https://www.lifeextension.com/vitamins-supplements/item01534...