Xiaomi 12T Android 15-based HyperOS 2 update is now rolling out. The release is in Europe and seems exclusive to Mi Pilot members. It will take a few days to start rolling out for everyone.

Xiaomi 12T was released in 2022, running Android 12. At that time, the T-series was only eligible for three years of Android OS upgrades and four years of security support. With the device running Android 14 and getting a new upgrade to Android 15 with HyperOS 2, it means the OS support has ended.
Details of the Xiaomi 12T Android 15-based HyperOS 2 update
Xiaomi is rolling out a stable OS2.0.1.0. VLQEUXM. The update size is 5.8GB and comes with all the expected features. It includes Alive Design Philosophy, Xiaomi Hyper Core, optimization, and bug fixes. Unfortunately, the device is not eligible for HyperAI, and we doubt Circle to Search will be available after this upgrade.
Nevertheless, there has been an improvement in animation. Security has been enhanced, and with the January 2025 patch, which is coming with the update, security vulnerabilities have been patched. We recommend installing the update, though it might take a few days before reaching your device.
What happens after the Xiaomi 12T Android 15 upgrade

Xiaomi 12T is only eligible for three generations of Android. Once you install the Android 15 build, support for Android OS stops. The device will receive only security patches moving forward, and that will end on August 8, 2026. Perhaps you should start saving to upgrade to a new device next year.