Can’t say 100% every handset because I not see every handset. The protocol for handset power off is different from low battery power. It needs to be different as user could connect handset to PSU when power low. When battery drain the handset sets in motion a recovery process to protect memory, close down any running apps, save memory and PIN management etc. To burst to the network with a packet to inform BTS / VLR of IMSI detach wouldn’t be relevant – as AFAIK.