It was discussed when the new setup wizard was being developed.
Its something we considered but would require us to test for any cell network leaks that may occur as devices boot. There is some chance there may be a cell leak before the OS is fully booted, particularly when a device may not have had airplane mode activated on the previous boot (eg. before being flashed with GrapheneOS). The cellular baseband may start functioning before the OS is ready to tell it to switch off.
This testing would be required for all supported devices. Should probably be a retest if there are any changes to the broadband firmware. We do not want to falsely suggest that transmissions are blocked.
There is interest in doing this but we do not currently have the equipment or the time to do it.
Also another thing worth considering - for many people what value is added with this?
In most cases the device will be on stock before flashing GrapheneOS. After acquiring the stock device it has to be booted and in most cases it will not have airplane mode active. Also in most cases need to connect to network to enable OEM Unlocking. If you want to be super private/anonymous there are a lot of awkward maneuvers required to acquire the device and flash GrapheneOS. Having airplane on by default in GrapheneOS only provides a small assistance.
If they acquire a device with GrapheneOS already flashed there could be benefits for people who intend to operate mainly, or always, with airplane mode on.
Also for anyone who operates in airplane mode who wants to do a factory reset.
In the mean time there is also the option to find a basement or somewhere out in nature where there is no cellular network coverage.