Using plantuml 1.2018.10 and the example below, setting the skinparam packageShadowing to false doesn't influence the appearance of a package in the generated image. This behaviour is in the class diagram. The behaviour for a stereotyped package is the same, it doesn't;t influence the generated image.
@startuml
skinparam packageShadowing false
skinparam packageShadowing<<Rectangle>> false
package Package1 {
class Comp1
}
package Package2 <<Rectangle>> {
class Comp2
}
@enduml