Using plantuml 1.2018.10 and the example below, changing the shadowing of a stereotyped package doesn't influence the appearance of a package in the generated image. This behaviour is in the component diagram.
I noticed setting shadowing (true/false) is also not working for non-stereotyped elements, e.g. cloud, package, database, but IS working for component.
The example has shadowing disabled for the stereotype <Layout>, but changing all packageShadowing to false and of the stereotype to true doesn't change the end result.
@startuml
skinparam packageShadowing true
skinparam packageShadowing<<Layout>> false
package Package1 {
component Comp1
}
package Package2 <<Layout>> {
component Comp2
}
@enduml