Sell by is entirely unrelated to quality though. It's just how often they think they can get away with. Like the recommendation on a box of baking soda... Put it in your fridge and change it every 30 days. No thanks.
It does seem to go “flat” after a few months regardless, even when the expiry date is 2 years in the future.
Then they switched to 30 days recommendation to goose sales.
A sell by date makes perfect sense on salt. No packager wants to be responsible for material that sat on someone else's shelf for years and years.