They will most likely be stuck on Oreo
While the ZenFone 4 has been updated to Oreo and the first ROG Phone is currently running Oreo, both phones might not receive Android Pie in the future at least what CNMO.com has reported.
According to the report, ASUS released a statement that the ZenFone 4 (ZE554KL) will not be upgraded to Android Pie in spite of releasing Android Pie Beta for the said device, citing “system stability and energy considerations”. While ZenFone 4 users can avail of Android Pie beta, ASUS warns that the said build is not stable and that it will not receive the usual system updates.
As for the first ROG Phone, ASUS promised a 3Q 2019 timeline for the Android Pie rollout though a recent post at the ZenTalk forum confirmed that the update is postponed indefinitely.
All hope is not yet lost for ROG Phone 1 users, as ASUS is working on the kinks before officially rolling out Android Pie. For the meantime, ASUS is rolling out security updates for the ROG Phone 1.