Thank you for asking. You may follow the steps below as 10 to 9 downgrade steps.
1. Download "Version Android 10 downgrade to P" and then put the image file into the root directory of the MicroSD card.
https://www.asus.com/mobile/phones/all-series/ZenFone-5/HelpDesk_BIOS/2. Insert MicroSD card into phone and then power off the device.
3. Long press with "Power" key and "Volume Down" key together and stop after feel the vibration to enter Recovery Mode
4. Use "Volume Down/Up" key and press "Power" key to choose "Apply update from external storage"
5. Select the downgrade file
6. When the updating process is done, Choose "Wipe data/factory reset" and tap"Yes" to perform the master clear.
7. "Wipe cache partition" > "Yes"
8. "Reboot system now"
#Android 10 to Android 9 Guide
https://www.asus.com/support/FAQ/1042349
