The changes to the page were made yesterday, which is when we assume the rollout began. As per the changelog, the update, which weighs over 1.6GB, also fixes compass calibration, adds a new e911 call flow, BYOD, and a new security patch.
As usual with OTA updates, T-Mobile seems to strongly indicate that this rollout is staggered, meaning some users may have to wait for several days before the download notification arrives.
The Note 9 update on AT&T and Sprint came with build number CSAA, but the T-Mobile version has build number CSB3 and the firmware seems like for the unlocked model (N960U1UEU1CSB3).
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