They have treaties with some countries so that if you don’t pay you can have your wages garnished. This is assuming you have citizenship and not residency etc with the other country.

So it’s on a per country basis. In some, nothing happens. In some there’s also treaties so you don’t have to pay double tax anyway, or it’s percentage based.

However, this also assumes you have no family you care about anymore in the USA or that they’re able to travel to see you instead. Otherwise you face prison for tax evasion if you ever go back.