Samsung Galaxy S8 Plus was announced last month along with Galaxy S8. Now its getting a new update with April month security patch. Though, the update currently available in some European countries Belgium and Germany.
The new update comes with build NRD90M.G955FXXU1AQDD and size is not confirmed. If you are leaving in above mentioned countries and still did’nt get update push notification than you can manually check by visiting Setting > Software Updates . Usually, OTA updates take time to reach targeted audience.