Samsung devices haven't historically been the quickest to receive OS updates. This week, Android 9 Pie came to the T-Mobile variant of the extra-durable Galaxy S8 Active, nearly a year and a half after the Oreo update.
The update, which has baseband version number G892USQU5CSG7, comes with all the normal Pie improvements, like optional two-button navigation. It also includes the July security patch, as well as "other improvements," according to T-Mobile's support page. The update comes in at about 1.7 gigabytes.
If you're using an S8 active on T-Mobile, you should already have the update — if not, go to Settings > Software update > Download updates manually to initiate the download.