Margarita Salt does not ever go bad and can be safely used for many years without any health issues.
The only that would likely happen to salt past it's use by date is that the salt becomes hard but you could grind the salt up or smash it up and still use the salt once it's hard.
Salt is old anyway and has been in the ground for 1,000s of years before it is mined so it can never expire like other foods.
Margarita salt as well as many other types of salt can be used for years without needing to throw it out.
The use by date on the salt is just a guideline as to when it's freshest after packaging but you can safely eat the Margarita salt many years past it's use by date.
Salt unlike other foods can never spoil.