Asus promised the Android Pie updates for their phones quite a time back, but missed their deadlines. After that, Deepak Sharma, head of mobile business, Asus, took it to YouTube to apologize for missing the timeline for the updates from the users and revealed new dates for the roll-out. He announced that the Zenfone Pro M1, Pro M2, and the Max M2 will get the updates by April 15th. Beta versions of the update were out since a few days, but you had to fill a form, deal with bugs in the OS, and report them to Asus. Not everyone was interested in this, so people instead waited for the stable update.
The update for the Zenfone Pro M1 and Max M2 started rolling out yesterday, and Pro M2 users have started receiving the update (PKQ1.WW_Phone-16.2017.1903.061-20190401) today. This update like the ones for other 2 phones is being rolled in in India only as of now. Global availability has not been announced yet. Also, as it is an OTA update, it is being rolled out in phases so chances are there you might have not still received it. You'll get a notification as soon as you get the update, or you can manually check it in Updates inside Settings.
Surprisingly, the update comes with a security patch of March instead of April. Also, you'll not notice any visual changes in the update. The Android 9 Pie navigation gestures are also missing. Nor are any full-screen gestures present in the phone. Asus might add them in future updates. Once the major update has been rolled out, new features can also be added by some minor updates in the future.