US carriers have needed a little more time to start delivering their Android 10 goodie packs to these 2018-released powerhouses, and one mobile network operator in particular appears unable to properly convey status reports and schedules to its customers for some reason.
As described by a disgruntled Galaxy S9 user on Facebook, T-Mobile's Android 10 rollout for this specific model and its plus-sized sibling has been a "comical PR disaster", starting with the fact the update was never listed as "in development" or "testing" on the two phones' dedicated software support webpages prior to its completion. Then, all of a sudden, the "Un-carrier" listed the Android 10 update as "available on February 23" for both the Galaxy S9 and S9 Plus.
The only problem is the over-the-air rollouts never actually began as far as we can tell. After a few days of reportedly instructing some S9 and S9+ owners to perform factory resets while simply telling many others they were looking at a phased rollout scheduled to be completed by March 6, T-Mo quietly revised its support webpages to essentially confirm the updates are indeed not taking place right now.
Curiously enough, Magenta seems to insist that was the case at some point, with Android 10 deliveries now "temporarily paused" and coming "soon" to all Galaxy S9 and S9 Plus users. While it's entirely possible you're only looking at a short delay here, people are not happy, primarily because of how poorly T-Mobile has communicated with its customers in the last few weeks.
On top of it all, T-Mo is already the last major US carrier standing without a wide-scale Android 10 rollout for the GS9 duo. That's right, Sprint has reportedly managed to beat its future sister brand to the punch earlier this same week, following AT&T and Verizon's suit. For what it's worth, the Android 10 update for Magenta's Galaxy Note 9 is also listed as "available" since February 23, and in that particular case, we have a bunch of actual user reports confirming the rollout is indeed underway.