Silent Hill: Downpour disappeared from Xbox Live and is no longer available for purchase. The reason behind the delisting is currently unknown, but the recurring theme of license expiration could be the reason.
Vita game Silent Hill: Book of Memories and PSP game Silent Hill Origins are unaffected. The PS1 Classic version of Silent Hill remains on PSN as well. Xbox Live, oddly enough, still has Silent Hill: Homecoming (which is only physical for PS3) and Silent Hill Collection available for download. Only Downpour is affected. Silent Hill: Homecoming is also still available on Steam.
The various changes could be related to licensing, as what happened with other games that got delisted recently. Either way, if you want to play Downpour, you’re going to need to settle for a physical copy.
[UPDATE 1/5 12:09 PM PST] Our original headline read that Downpour was delisted from both Xbox Live and PSN, but the game was never in fact available on PSN. The game was indeed delisted from Xbox Live today, however.