Samsung may not have the best reputation for keeping all of its phones on the latest OS versions, but it's usually pretty good with its latest flagships. Case in point: an update with the May security patch, as well as improvements to Bixby (no, it's not Bixby Voice) and microSD card stability, is being pushed to the T-Mobile variants of the Galaxy S8 and S8+. Sprint models are also receiving an update, and we'd expect it to contain the same things.
So, what exactly is new here? The security patch has been upped from April's to May's, and there's reportedly improved stability for microSD cards. However, although that mention of Bixby improvements may have excited the dozen of you that actually use Bixby for the arrival of Bixby Voice, it really just adds shortcuts to the top of the screen for Bixby Vision and Reminders. It is possible that the update is adding support for Bixby Voice in the background, but it still hasn't been enabled.
The update weighs in at 375.60MB for the T-Mobile plus variant, and we don't imagine that number being drastically different for the standard T-Mo S8 or Sprint models. If you haven't already received a notification, manually checking for it in settings should make it appear. Neither Sprint nor T-Mobile appear to have updated their support pages yet.