The wait will finally be over, as MIUI 9 Global Beta ROM 7.8.10 is arriving this Friday. The first batch consisting of Xiaomi Redmi Note 4X / Redmi Note 4 Qualcomm, and Mi 6 will receive the update.
The existing Beta ROM users will automatically receive the update through OTA (over the air updates). Meanwhile, the stable users have to unlock Bootloader and perform a full wipe before Fastboot.
The remaining smartphones are:
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Source (MIUI Forum)