I am working with android phones at work, we want to remove all google accounts from the phones before we give it to our customers.
I know that the phones are not able to receive app updates, but what about os update from android?
OS updates are provided by the device manufacturer. They are retrieved directly by the device and don't require any user account.
The only requirement for an OS update is a working Internet connection and enough free space on the device.
Adding to the answer Android plans to provide security patch updates through play store in future called project "Mainline", so with advent of android Q you will probably need to have a Google account to install at least security updates. Also Google has been trying hard to streamline the OS update procedure across vendors so you never know. Hope my answer added some value.