Update: US carriers have also started rolling out the Galaxy S8 and S8 Plus update with December security patches. Starting with T-Mobile, the Galaxy S8 security update is rolling out with firmware build G950USQS2BQL1 and G955USQS2BQL1 for the S8+.
The official changelog from T-Mobile only mentions December security updates. However, the new update should also improve stability for both the devices and fix some of the minor bugs/issues as well.
The new update includes the latest December security patch. Apart from that, there aren’t any new noticeable changes or improvements. Since it is a security update, we recommend that you install it right away. As of now, only the Korean variants of the phones are seeing the update, but other regions should be getting it very soon as well.
The build number for the Galaxy S8 is G950NKSU1AQL3, whereas the Galaxy S8 Plus gets build G955NKSU1AQL3. Samsung recently pushed the 4th Oreo beta update to these devices in Korea, US, and the UK. A stable Android 8.0 Oreo update for these devices are still about a month away.