Xiaomi follows a different release cycle for its updates, wherein the MIUI version of the device has more power to dictate the featureset of the phone, more so than the Android version. This means that base Android versions are not as important to the overall working of the device, and these updates may get lower priority. On the other hand, the company did consider user feedback and decided to work on Android Pie updates for the Mi 6, so that's a good sign (although we aren't sure if the entire exercise wasn't merely a marketing activity).
Now, other newer devices like the Xiaomi Redmi Note 5 Pro, Xiaomi Redmi Note 6 Pro and the Xiaomi Redmi S2/Y2 are on track to receive their Android Pie update. All of these devices have already received a MIUI 10 update (here for the Note 5 Pro, here for the Note 6 Pro, here for the Y2), so this new update will mostly be for bumping up the Android version number on these devices, rather than for new features.
Xiaomi Redmi Note 6 Pro XDA Forum Xiaomi Redmi Note 5 Pro XDA Forum
Xiaomi Redmi Y2/Redmi S2 XDA Forum
To provide this update, Xiaomi is looking for beta testers for these devices, who would be willing to install a MIUI Global Beta ROM on their device and give feedback to the development team. These beta testers can be from any region as long as they can communicate in English and use the QQ messaging platform. Of course, you do need to own one of those devices in the first place, and have the ability or inclination to unlock the bootloader and flash a MIUI ROM; as well as be willing to share feedback about bugs since the whole point of a beta exercise is to figure out bugs before a wider, stable release. Naturally, there will be bugs, so stay away if you are looking for stable, daily driver software. There are a few incentives for beta users, but these revolve around additional privileges in the Mi Forums. Curiously, if a user posts three bug threads without permission from a Moderator, "he will be removed from Beta team immediately", which in our opinion, is a ridiculous condition/reaction.
The application deadline is March 15, 2019. You can read more about the recruitment and the application process in the Mi Forum thread.