Got this in a roll Friday - it's probably acid dipped or altered in some way as I can't find anything about a matte finish half. Then again the input of Kennedy half collectors is appreciated.
How about "sand blasted"? It doesn't really matter though since the coin has been horribly damaged post strike.
The 1998-S Kennedy was produced with a matte finish. It was made specially for the 2-coin set which contained the Robert Kennedy $1 Commem. Your specimen looks like it may have been dipped in an acid as you suggested. Chris