Every time someone uses the word “decimate” in the sense of “severely reduce”, a raging language purist appears out of nowhere explaining that “decimate” comes from Latin decimare, which refers to a Roman practice where sometimes as a punishment, every tenth member of a rebelling military unit was killed.
The raging purist’s etymology is correct, but implying that the word was used improperly is inappropriate. The original meaning of “reducing by a tenth” is now so uncommon in literature (apart from literature discussing the actual historical practice, of course) that it may be well considered obsolete. If you use “decimate” to mean “reduce by a tenth”, you will most likely be misunderstood.
I even dare say that the purist’s belief is based on a misunderstanding. One tenth doesn’t sound like an awfully lot, but imagine you were in a group of 100 people out of which 10 were to be randomly chosen and killed. You would be dying of fright. When it comes to the risk of death, one tenth is a huge number. That’s why when people thought of decimation in its original sense, they still understood it as “killing off a huge part of a group”.