There's no Greyhawk material yet for 5E
Unfortunately, no material for any setting except the Forgotten Realms "officially" exists yet for 5E.
However you do have a few options:
- Vecna Lives: This AD&D 2E adventure could be updated for 5th edition (though I'm not sure how much work it would take, I haven't looked at it for a while). It does provide a little blurb about the history of the sword, and if I remember it right, the players do encounter the sword.
- Book of Artifacts: While this supplement (I think also for AD&D 2E) contains a page and picture of the Sword of Kas, it doesn't really provide much about where the sword is now, even for the time period of 2E. However, it provides some good suggestions on how to use it.
TL;DR: There really isn't a lot of history on what the Sword of Kas is doing currently. It seems that the official publishers have included it in a few books, but they really want DMs to decide where it is and why, as is the nature of most artifacts. They are powerful items with legendary histories, but most of them are fairly easy to incorporate into your own campaign. There's no real answer for where the sword is in the c. 1490s DR (5E timeframe, probably).