My understanding has been that the battery has to be removed to end all radio transmissions, short of the barrier method they are suggesting.
From the movies...don’t you want to remove the SIM card..or is that only so you won’t be tracked
‘One feature of Airplane mode is that it saves power. Because the device doesn't search for a network signal as in other modes, the battery lasts longer.’
Which is why I started using Airplane mode, but I also have free battery doctor.
There is a reason we were not allowed to bring the danged things into any controlled access areas of a facility. It didn’t matter of they were on/off, batteries/SIM cards removed. The only way to be sure was to not have them around.