I've written previously on the historical documentation of decimation here. Even in the classical era, we know it was considered a particularly harsh, infrequent punishment, especially during the Empire:

• It was reserved for cases of extreme cowardice and refusal to follow orders. Consul Appius Claudius ordered the earliest recorded decimation when an entire group of troops refused to march.

• During the late Republic/Civil War, some folks, as a form of mud-slinging, called out their political opponents for using it.

• Accounts where we see the actual word decimari, "to be decimated," are about the emperor Galba, whose 7 month rule was more about restructuring the economy with fire rather than sense.