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A possible workaround is to do something like:
@startuml Bob -> Alice : hello == start of deprecated == Bob -> Alice : hello deprecated == end of deprecated == @enduml
http://www.plantuml.com/plantuml/uml/SoWkIImgAStDuNBAJrBGjLDmpCbCJbMmKiX8pSd9vxArLG0hKCXFKqX9BIXAJKuiIKrHiBN5gXn92KXdQbuAfZuk1w0mq3u0
Far from perfect, but at least it works!