grammatical number - How do you use plural or singular form of an adjective?

So I was taught that you should almost always use singular form of an adjective instead of plural form. But recently, I've seen people using plural form and it sounds quite weird and I don't know if it is correct.

For example,

• "Salary tax" instead of "Salaries tax"


• "Office supplies" instead of "Offices supplies"


• "Product demo" instead of "Product demo"

In the above cases, the word "Salary" is used as an adjective to describe the type of "tax", so it will be in singular form. But I saw people using salaries tax and not sure if it is grammatical.

So my question is if there are any general rules on how to use the singular/plural form of an adjective?