The Galaxy S8 and S8+ on Verizon, AT&T, and Sprint are receiving February security patch, that doesn't include Android Pie and yeah brought disappointment to many users.
This latest update for Galaxy S8 and S8+ brings February 2019 security patch that provides fixes to 12 Samsung Vulnerabilities and Exposures. Also, the update installs bug fixes and stability improvements.
This update for Galaxy S8 and S8+ is rolling out with software version G950USQS5CSB1 and G955USQS5CSB1 respectively, same for all three carriers. If you want to check this update, please head into Setting > Software Update.
With that being said, T-Mobile may also send the update to these devices but the carrier always takes enough time on the topic. As the Pie update could be expected anytime this month.