Samsung Galaxy S8 and Samsung Galaxy S8+ on T-Mobile received November security update in the second week of December. Now T-Mobile has started to push December security for Samsung Galaxy S8 and Samsung Galaxy S8+. The software versions will be bumped to G950USQS2BQL1 for the S8, and G955USQS2BQL1 for the S8+. The update weighs 130.66MB for S8 and 127.11 MB for S8+.
The update will be rolled out in phases, it may take couple of days for some users to receive it. Update can be manually download by going to Settings > Software update > Download updates manually. Once the update is downloaded make sure that mobile has enough charge to update.