The whole point of transponders is to allow air traffic controllers and automated anti collision systems to prevent two aircraft occupying the same area of sky at the same time. The fact that ADSB transmissions can be picked up and decoded using a £5 TV dongle and a PC running appropriate software, or a mobile phone app, is irrelevant as far as air safety is concerned.
If a determined effort is made to target high profile individuals, there is no need for high tech methods. Simply observing a place they are known to be attending is sufficient. Computer assisted tracking doesn't necessarily increase the security risks associated with holding a high profile public office.
Switching off or spoofing an aircraft transponder will increase the aviation safety risks far more than it decreases the chances of a personal attack on an individual flying in that aircraft.