Plane hijacking was an every decade occurrence. So there there isn't enough data to say if they have made a difference yet (this is debatable, but there at least isn't enough data to have high confidence). That is before we get into confounders: the economy is different. The 9/11 hijacking changed the game so potential hijackers are less likely to try since passengers are no longer going to sit back expecting a free trip to Cuba. cockpits are not locked in flight, so a hijacker couldn't do anything even if they tried.
None of the TSA screening changes would have made a difference. The knifes the hijackers used were placed on the planes ahead of time (the old metal detectors would have found them). Checking IDs against a no-fly list could be done in many less intrusive ways.
This is well before TSA but plane hijacking used to be common occurrence: "Between 1968 and 1972, more than 130 American airplanes were hijacked. Sometimes there was more than one hijacking on the same day. "
None of the TSA screening changes would have made a difference. The knifes the hijackers used were placed on the planes ahead of time (the old metal detectors would have found them). Checking IDs against a no-fly list could be done in many less intrusive ways.