That's unlikely because people would notice the devices working while off. Your power draw comes from two sources unless you run games and heavy apps: display and radios - and there's enough interest and measurement happening there that people would notice radios activating when they shouldn't.
There's also lots of interest in tracking device communication and I really expect someone to notice a randomly appearing device where there shouldn't be one.
The attack scenario is, that the mobile just listens to you via the microphone and saves it - and later when normaly turned on, sends away all the data.
All of this on a very low hardware layer, so no need for complex cpu operations or engage with the OS(in case of turned "off"). So very low power demand.
And it would also not show up, in anyone doing radiotraffic/wlan analysis.
So it would be indeed very hard to spot.
(don't have the sources, but I think on some defcon was a talk with proof of concept about this)
So if anyone thinks, he is a specific target of some powerful intelligence agency, (like someone strongly engaged with the opposition in Hong Kong) - I think they definitely should consider this scenario as a possible one (but I don't know how likely it actually is, probably not high, if your are not considered a leader).
But that this change for non-removable batteries in general was made, so that even the paranoid part of the population can be tracked non-stop by the global Illuminati ... is indeed very much tinfoil area.
But the part about your phone maybe spying on you, when you think it is off:
There's also lots of interest in tracking device communication and I really expect someone to notice a randomly appearing device where there shouldn't be one.