Airliners flying into skyscrapers will always be seen as related to the 9/11 attacks.
It is interesting to see how the view on this has evolved. They used to say "Never forget" and here we are.
In my online experience 2001 to 2010 such humor was not received well. It probably was popular on 4chan and other degenerate places.
But if someone posted 9/11 memes in the "Funny Picture Threads" of normal forums then other users would react negatively to it. Always. It still is like in some forums I read. I has changed slightly but not much. At worst you have the "jet fuel can't melt steel beams" meme maybe. But it is a thin line, that meme is more about the investigation and conspiracy nutjobs. The attacks or deaths itself seem taboo in "civilized" online communities.
It is not even because of moderation, people just avoid it.
On bigger, more anonymous platforms (youtube, imgur) it seems more accepted. There you have videos that remix footage of the impacts or falling people.