Asus has recently revealed a detailed Android Pie update roadmap that includes a wide range of handsets that mainly belong to the Zenfone 4, Zenfone 5, and Zenfone 5 families. However, the official ZenTalk forum topic is currently under moderation (or has been removed), so we should expect some changes to the list mentioned above.
https://www.notebookcheck.net/Asus-publishes-Android-Pie-update-roadmap.410995.0.html