I wouldn’t say archaic or historical definitions are improper, but you’re right - the primary meaning of decimate in English changed and now means to destroy the majority of. Maybe this is because decimate was always very damaging; threat of death is very serious, regardless of the numbers.

That said, I had sweet breads recently. And a cat being a deer sounds strange in English now, but deer is still the word for animal in other Germanic languages today, even if it faded in English, so it doesn’t sound completely archaic.