Are there countries which post-9/11 have adopted legislation allowing the shoot-down of passenger aircraft under any circumstances ?
Yes it's legal in some countries/locations.
In the US after Sept-11 Operation Noble Eagle expanded the US Air Force's role to providing air defense for the entire US domestic airspace. Flying into a Special Flight Rules Area (or whatever they are calling it this week!) such as that over Washington DC and or into the even more restrictive Washington, D.C. Metropolitan Area Flight Restricted Zone (DC FRZ) without adhering to the Zone's rules and failing to respond to communications and intercept protocol can (in theory) result in being shot down. The DoD (understandably) doesn't reveal details on the exact rules of engagement or who has the authority to order such an action (believed to be POTUS, Sec. Def and Deputy Sec. Def) but it's definitely possible (and legal), and they've come close. While the near-downing in the link was for a private aircraft the same rules would allow for the shooting down of a hijacked commercial aircraft.
Internationally speaking.. well it's a grey area. As discussed in this paper there is no International Law that specifically prohibits doing so - although it's generally seen as a bad thing to do except as a last resort. So as Ron Beyer mentioned in a comment, there generally doesn't need to be any legislation enacted to do so - because there isn't any that prevents it.