So, no sources. Got it.

The original Greek “-ikos” was both the feminine singular when refering to “the art” (the whole field)

In modern English it's The Arts - plural as it refers to all types of art (music, painting, etc.).

whether the s was thruncated appears random

I'm not sure North Americans would appreciate being called "random". 😂 Just the other day I was surprised when I saw a Canadian who used an American spelling, and when I asked him about it he said he was pretty much forced to because programming uses American spelling.

useage

Usage

several thousand year old grammar (from a region remote enough that we forgot about it for several centuries) with syntax rules not present in the original.

Did you miss the part where it says it's a borrowed word?