T-Mobile starts updating the Samsung Galaxy S8 and S8+ to Android 9.0 Pie
Don't worry if it hasn't yet popped up on your phone, as T-Mobile, just like the other carriers, is deploying the update in waves. Although it might take up to a few weeks, all T-Mobile Galaxy S8/S8+ units should eventually receive the update.
As one would expect from such an important update, you'll need around 2GB of internal storage room available, as well as at least 50% battery. The update weighs in at 1.6GB, Droid-life reports, and includes One UI version 1.0 along with March security patch.
Of course, all the new features and improvements included in Android 9.0 Pie will be added as well. Keep in mind that this is likely to be the last major Android OS update that your Galaxy S8/S8+ will receive. If you want Android Q, which will be released in about 5-6 months from now, you'll have to switch to a newer smartphone, preferably another flagship.