The fact that it used to be somewhat more typical (though not everyday occurrence) does not make it less of terrorism. That definition does not depend on freque

The fact that it used to be somewhat more typical (though not everyday occurrence) does not make it less of terrorism. That definition does not depend on frequency of an event. It depends on the use of terror to achieve political means, and a mass beheading definitely tells the vanquished people that from now they'd rather obey their new overlords. It is a matter of linguistic taste if we use such word in medieval context, but the principle is the same as in modern terrorism, and jihadists of the 21st century explicitly say that they are motivated and inspired by Muhammad's acts like this one.