I think the issue here is, it depends on the handset in use. According to 3gpp specs, it is not mandatory to send an IMSI DETACH message when the battery is low. That does not mean that if a manufacturer wants to include it they are forbidden…they are free to, as long as they stick to the stipulated protocol for handset power off. In summary, it is possible to have a handset sending an IMSI DETACH message, depending on manufacturer, but it is not critical for handset designers.