I don‘t think that is true at all. There have been numerous hijacked planes since 9/11 including two in the USA just this decade.

https://en.wikipedia.org/wiki/List_of_aircraft_hijackings

Plane hijacking has been on its way out anyway after the turmoil of the 1970s. And that has probably more to do with a) the relative political stability of the post cold war period, and b) a general sense that airplane hijacking isn’t actually that likely to advance your political goals. If you read the list above, you see people hijacking planes all kinds of dumb methods, hardly any of them involves carrying an actual bomb onto the plane.

> a) the relative political stability of the post cold war period

Most plane hijackings/bombings were middle east related (e.g. linked to one of Palestinian liberation, al-qaeda, or isis)

Not sure i'd call that a stable region of the world, especially now. Perhaps though the people involved just realized it was an ineffective strategy.

There has been way less terrorism in general too. I'm always curious whether the war on terrorism is that effective, or there's major socioeconomic factor that matters most (or there's just less lead in the air).