We found these weapons and all the others we KNEW they had except for a couple, anthrax and something else IIRC; we just didn't find the ones we THOUGHT they had and almost certainly never did. They are still highly dangerous, which is why they have not yet been destroyed. Iraq has to build a facility and have it certified before they can begin destroying them, else they would surely kill the workers and perhaps the general public around them. My brother-in-law works at one of the facilities which recently destroyed chemical weapons including Sarin and VX weapons manufactured just after World War II as well as some others (phosgene and mustard gases IIRC) dating from World War I - although again if I remember correctly, his facility merely dissembled and repackaged components which were actually destroyed elsewhere. Russia did the same. I think we both finished in 2012 or 2013 if memory serves. These things have a loooong shelf life, but an important part of maintaining them includes periodic inspection (X-ray IIRC) to make sure the shell hasn't corroded to the point of rupturing. With our stock, inspections were done every ten or fifteen years if memory serves and some small portion always had to be discarded. I doubt Iraqi technology in the 80s was any better than ours in the thirties, so I'd assume a significant portion of these would no longer be viable and another significant portion would fail if used conventionally. I know the CIA publicly stated that some Iraqi-manufactured Sarin (that made with Iraqi-made precursors which were less pure than the French precursors and those weapons built as pre-mixed rather than binary-stored) had an effective shelf life of no more than a few months, so it's possible that these things are much less toxic than designed.
As far as filling a plane with them, likely that would be exactly as effective as the plane itself. To be effective, chemical agents need to be spread reasonably evenly over a large area using a means that does not destroy the chemicals. The Iraqi insurgents tried a couple times using IEDs to spread chemical heads; the blast mostly destroys them. An airliner crash would likely do so even more unless they managed to run it out of fuel, in which case we'd have contaminated ground rather than an effective Sarin attack.
I'm not any more thrilled about having chemical weapons in the hands of the Religion of Peace's most fanatical supporters than I am about having airliners there, but I'm guessing they are more likely to kill themselves than us with the former. Just because something remains highly dangerous doesn't mean it remains an effective weapon.