Platies don't need salt but they can tolerate it.
There is a lot of misinformation out there about livebearers. So many people say that all livebearers and especially mollies need salt. They don't. They need hard water.
I think the confusion stems from the word salt. In chemistry, a salt is the product of the reaction between an acid and a base. Common salt, sodium chloride, is the product of hydrochloric acid and sodium hydroxide. Calcium carbonate (which I have mentioned many times in relation to water hardness) is the product of carbonic acid and calcium hydroxide. Nitrates are salts of nitric acid, nitrites are salts of nitrous acid. Sulphates are salts of sulphuric acid. On the base side there is sodium, potassium, calcium, magnesium etc.
As you can see, in chemistry there are many salts and a lot of these occur naturally in fresh water. One that doesn't is sodium chloride, aka common salt, the stuff you put on your chips.
So if I were to say that livebearers need salts in the water, almost everyone would interpret that as needing common salt when what I actually mean is water with lots of salts in the chemical sense, that is - hard water.
Just for completion, several molly species are from brackish water areas. But the mollies in shops bear little resemblance to wild mollies. Shop mollies are usually hybrids of more than one wild species, and they have been bred in farms for so long that they no longer need any common salt at all. But mollies tend to suffer more than any other livebearers if they are kept in water that is not hard enough.