Just my opinion, but I think modern muscle starts about 1986.
It's pretty subjective, but my definition of "Modern Muscle" means decent power (or better), EFI, OD, AC, Cruise, Power Windows, bigger brakes and handling, etc. Certainly there are gray areas, much like the definition of a "Muscle Car" is '72 and older even though the low compression motors started in '72 and single exhaust and cat's didn't start until '75.
So when the '86 Mustang GT came out with modern EFI and had a decent amount of power, it fits in my mind as the start of the "Modern Muscle" era. Note that to me "Modern Muscle" covers what the factory offered, not what someone could build. So a '73 Duster with a 5.7/T56/AC doesn't make it modern muscle even though it carries a modern motor. But an '82 Mustang with a Coyote swap might, but only because the chassis is the same as an '86. Lot's of gray.