I still don't have the device to test this my own and very wonder if the Magisk root is preserved during OTA like in the good old days before Android 8.0, or if we want to enjoy the benefits of root, we must be prepared to patch the boot.img manually with every OS update? Has anyone checked?
Are you referring to OTA for LineageOS or OTA for stock (after bootloader unlock and root with Magisk)?
Another user answered for the former, so I'll answer for the latter (for dopinder, the ONN device): yes, with each OTA you will need to re-root with Magisk. This can be done either by flashing the Magisk APK (replace the .apk extension with .zip) by booting (not flashing) Lineage recovery (and requires a micro USB OTG Y cable so you can use a keyboard, as the hardware button won't work), or by extracting boot.img from the factory image zip and patching using Magisk once booted.
AFAIK, the second way doesn't really work right now because (again, AFAIK) no one has captured the OTA file for boot.img to be extracted. You can't set up a new ONN device without completing the setup wizard, which downloads the OTA before completion. Since you can't enable USB debugging (for doing a logcat and capturing the OTA URL) until after setup completes, it would take someone whose device is setup, has not received the OTA, and can do a logcat while the device updates in order to grab the OTA.
That leaves us with the first option of using a micro USB OTG Y-cable and Lineage recovery.
Something I don't know (which perhaps @npjohnson can comment on) is whether a stock OTA can re-lock the bootloader and and either brick the device (due to modifications that were done using root) or at the very least prevent us from unlocking again.