Last month ASUS confirmed that the 5Z will get the Android Pie update by the end of January 2019, and at the launch of Zenfone Max Pro M2 earlier this month, it once again confirmed the same. Today the company has started rolling out Android 9.0 (Pie) update for the phone in Taiwan, earlier than expected.
The update has the version number 126.96.36.199_20181222, is 1048MB in size, but it still has Android security patch for November, 2018. You can download the OTA Zip here and flash it manually, if you are running version 188.8.131.52 on ZS620KL model. Global roll out should begin in the coming weeks.