Hi,
Yes Jas will almost definitely be short for James but another name that dates back is Jasper and I have once seen that abbreviated as Jas in a parish register. I know it was the same man because I had seen him earlier as Jasper.
It is never wise to assume a name is a modern day invention, although I admit I have never come across Kylie or Peaches pre the latter half of last century. I have found Tracy as early as 1790s.

Jeremy