I've personally flown on private IFR flights where the flight plan was filed something like 10 minutes (or even less) before departure (or amended several times, etc). It doesn't seem like a good way to go about knowing where someone is. Everything regarding elonjet only applies to the aircraft and it's likely he loans out his aircraft to other execs/employees at his companies, miscellaneous friends and family, etc. So even if you somehow developed some methodical and resource-intensive master plan to attack the aircraft or movement associated with it you wouldn't have any guarantee you'd get your intended target.

A missile defense system was announced for Gulfstream aircraft at a cost of about $3.5m[0], which for a ~$65m aircraft isn't really that expensive...

[0] - https://www.aerospaceonline.com/doc/farnborough-gulfstream-o...