In sequence diagrams, shadow can be removed only on 'participant' and 'actor' types ; for the other types the shadow remains.
Reproduced on PlantUML-1.2019.06 & PlantUML-1.2019.12beta6
@startuml
skinparam participant {
Bordercolor none
Backgroundcolor none
Shadowing false
}
skinparam actor {
Bordercolor none
Backgroundcolor none
Shadowing false
}
skinparam boundary {
Bordercolor none
Backgroundcolor none
Shadowing false
}
skinparam control {
Bordercolor none
Backgroundcolor none
Shadowing false
}
skinparam entity {
Bordercolor none
Backgroundcolor none
Shadowing false
}
skinparam database {
Bordercolor none
Backgroundcolor none
Shadowing false
}
skinparam collections {
Bordercolor none
Backgroundcolor none
Shadowing false
}
participant "participant\n<size:30><&clock>" as participant
actor "actor\n<size:30><&key>" as actor
boundary "boundary\n<size:30><&beaker>" as boundary
control "control\n<size:30><&bar-chart>" as control
entity "entity\n<size:30><&circle-check>" as entity
database "database\n<size:30><&envelope-closed>" as database
collections "collection\n<size:30><&image>" as collections
@enduml
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