Well you can leave unsalted butter out but the chances of it growing microorganisms like mold is gonna be higher. Salted butter has, as you can guess, salt in it and that’ll reduce microorganism growth. Unsalted butter has acid added to it and the pH is low enough to inhibit some microorganisms from growing but it’s going to be a lot riskier, especially if the butter happens to have a lower than usual fat content which makes it even more hospitable to stuff that wants to grow there.

Whether it’s salted or unsalted, the high fat content of butter isn’t the most hospitable place for most microorganisms but some will still move in if they want, salted or not.

Salted butter is (generally) safer to leave out than unsalted butter but they’ll both go rancid at the same pace. Going rancid is usually going to be a bigger and faster issue than microorganism related spoilage.

Disclaimer: This is only in reference to butter made with pasteurized cream. Never leave raw cream butter out.