I can find some info about the sword and the fight with Kas and Vecna.
But where is Kas' Sword these days? Is there any official information about it?
Who got it after the fight? Where did it get lost after Kas died? Who holds the sword these days?
Best Answer
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:
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).