Yes it has already been out for S8 devices. If you haven't gotten the update, have you tried to manually update the device by going to settings. If it doesn't show up there try using Samsung Smart Switch on PC and see if the update shows up. If not can you say if the device is a Carrier device or Unlocked device and if Carrier what Carrier are you currently using.
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There are no prerequisites. I would follow the advice provided by NBI. If you are unable to update through Settings, you can try Smart Switch. If that is unsuccessful, you need to speak with your carrier. https://www.samsung.com/us/support/owners/app/smart-switch