There’s only a select number of devices on the market that have an official build of Android Pie available. These have come from dedicated OEMs who are working with Google and keep their eyes on the community and its demand for the latest version of Android. Then we have the custom ROM scene which is comprised of volunteers who dedicate their time and talent into the scene. Like clockwork, as soon as a new version of Android is released fans of various custom ROMs begin asking when they will receive the update.
These things take time, and again, since this is all done from their generosity (when is the last time you paid for a custom ROM?), it’s rude and annoying to ask for ETAs. We do generally see a surge of pure AOSP ports released for various devices when a new version of Android is released, but these are missing a lot of features compared to custom ROMs. That’s why it’s such a surprise when we see unofficial ports of a custom ROM like LineageOS 16 released for a device.
That’s exactly what happened earlier this week for the Xiaomi Mi Note 3 thanks to the work from XDA Recognized Developer dianlujitao (who is actually an official LineageOS team member). It is noted that this release is an experimental build with the developer calling it Alpha quality as of writing this. There are a lot of features that work with this build but you should be aware that the current known bugs list says VoLTE/VoWiFi does not work (and won’t get fixed anytime soon), and the build currently has SELinux running in permissive mode.
Source: https://www.xda-developers.com/xiaomi-mi-note-3-unofficial-lineageos-16-android-pie/