They wont add that feature because it wouldn't be robust unless implemented at a hardware level. Grapheneos assumes advesaries have encyclopedic knowledge of the OS so features must work in spite of that. It's already standard to put phones in faraday Containers because of remote wiping on some phones. It wouldn't take long for then to adapt and turn off the device to prevent the timer from functioning.

Although that would bring it BFU which is more secure. But there's already the reboot timer.