Al Qaeda had already mounted and claimed multiple attacks on the US, and even the World Trade Center for years before 9/11 and the Taliban continued to provide support and protection to the group and explicitly endorsed their activities throughout. They were clearly culpable.
It's not clear that they conspired [0], though I suppose you could call them an accessory after the fact for refusing to extradite bin Laden.
[0] https://en.wikipedia.org/wiki/Responsibility_for_the_Septemb...