The Android Pie update for the ZenFone 5 series is expected to arrive in January 2019 but an early version of the ROM is now available for manual download. The ROM appears to be intended for the Brazilian market but can actually be installed on any international variant. Those who have tested this ROM say that users will encounter a few bugs and they should rather wait for the official FOTA rollout.
https://www.notebookcheck.net/Here-s-a-first-look-at-Android-9-0-Pie-running-on-the-Asus-ZenFone-5.383756.0.html