At one time, it was possible to set visibility rules for custom fields. It seems like that functionality has either been eliminated or placed somewhere not obvious to me.

Has that feature been removed, or is it somewhere that is not obvious to me?

The use case here is having a Pay Type radio button, a Wage currency field, and a Salary currency field. When "Salary" is selected in the Pay Type radio field, then the Wage currency field would be hidden. When "Wage" is selected in the Pay Type radio field, then the Salary currency field would be hidden.