I absolutely hate to be the bearer of bad news on this one but..
Here's hoping it's just a rumor and there's no truth to it. However, if true, this is the saddest piece of VGM news I think I've ever gotten. I can't imagine Silent Hill without Yamaoka, it just.. can't be done. Silent Hill 2 is my absolute favorite game and that's mostly due to this man's brilliant music and overall sound design for the game. Here's hoping that if he does leave, he'll just go freelance and still come back for Silent Hill projects kinda like Uematsu does with Final Fantasy.
Ugh either way though. If Shattered Memories ends up being the final Silent Hill that he's involved in, I'm gonna cherish the hell out of it when I play, whether it's good or not.
Potentially the worst news ever for us true Silent Hill fans.