So there’s a Quora on this. https://www.quora.com/How-many-steel-buildings-have-collapsed-from-fire-besides-WTC
Apparently, yes, it’s happened a number of times. It doesn’t require jet fuel, burning offices is enough.
But it primarily depends on whether it’s steel or concrete. Apparently, concrete - and concrete coated steel - does pretty well in fires and rarely fails from the heat. But steel is enormously vulnerable, Quora shows a figure that indicates steel loses 90% of it’s strength at half the melting temperature.
So, in reality, conspiracy nuts are just nuts. If an aircraft slams into a building and starts a fire that weakens the structural steel of a building to 10% of it’s normal value, that’s enough. And in WTC, the girders were coated with a spray on material that had gaps, not to mention the aircraft impact no doubt damaged a lot of the fire protection. It also caused immediate structural damage to portions of the steel columns, just from the aircraft collision, further weakening the margins supporting the structure.
Anyways, the problem with a 1 kilometer structure is that concrete is far too heavy. At that height, you probably would need to use structural aluminum to save weight. And a quick bit of googling says that structural aluminum has less resistance to heat than steel does.
The only way I can think to do a mega-tall building and have it be fire resistant would be to use something semi-sci fi. Something like structural elements manufactured from the factory with a vacuum ceramic insulation. Vacuum ceramic (the stuff they use on space shuttle tiles) would be basically immune to the fire that could burn in a skyscraper, the problem is the stuff is very fragile and ironically would be destroyed from an aircraft impact.
Or, the other way, and this is the solution they had in the movie, it just didn’t work - would be active defenses. Basically building scale fire extinguishers. Not just sprinklers, special nozzles that can spray that fire suppressing powder that dry fire extinguishers have in them.