It was a while back, but I do remember airlines shut down. I think cargo flights got cancelled.
It was not as bad as the COVID lockdowns, but at the time the changes were pretty drastic.
A lot of overhead was added for security. Now you need ID to get into a lot of places you did not need before. For example: my uncle was a UPS driver, and he said before 9/11 people just saw the uniform and waved him through. Bottom line, I think 9/11 showed there was no slack capacity, and a lot of companies struggled. I guess companies just adjusted to the new reality.
I do remember reading articles/web pages questioning JIT, and stating that if there was more slack capacity adjusting would have been easier, although I cannot think of any specific URLs/writers at the moment.
I am just a software grunt, but I feel like I have seen a similar dynamic in every job/project I have been on. Management keeps yacking about innovation, and pushing more work on us, while dismissing the notion that more time and/or more people would make everyone's lives a lot easier.