It is technically a fallen government, yes; but it certainly has no place as it is today to claim any legitimacy. Fact of the matter is that the Taliban can be found in other parts of western Asia, most notably Pakistan; where they never held any legitimate claims in the first place, and commit acts of terror.
For that reason in particular I would denounce them as terrorists.
Another reason would be the fact that the Taliban, as I said before, no longer has any legitimate claims over its former territory. The government doesn't make the nation, the people make the nation. And the people have moved on; not necessarily in the best way possible, but they have moved on nonetheless.
As recently as resisting a cure to Polio, the Taliban has been involved in grotesque human rights violations and terrorism mostly against the people of Pakistan and Afghanistan. Fearing that the vaccine somehow opposed the Taliban's values, the Taliban has targeted and killed many health officials working in the are. In addition to this the Taliban is still violently oppressing women by opposing their right to receive an education and opposing their right to vote.
Before the Iraq war, Afghanistan was under the Taliban's iron and hateful fist. They were removed from power by the United States and its collaborators over time. They do not focus on hurting the U.S. directly as Al-Qaeda does, but more so on reclaiming parts of they're fatherland through attacking American troops with weapons that are more common with Terrorists than nationals.