Hello, please note that I'm Italian so my reports mainly concern the European Marketplace, but they could be worldwide.
XBLA Games Delisted:
Sega Rally Online Arcade (200G)
PRG4 - Free Challenge Pack (80G)
PGR4 - Premium Challenge Pack (170G)
(So basically every PGR4 DLC, with releted Achievements)
Also note, even if this is not releted with unobtainable achievements, that Sega Rally (Sega Rally Revo in USA), and Activision's 007 games (007 Legends, Blood Stone, Quantum of Solace and GoldenEye Reloaded) are no longer sold on GamesOnDemand. It could be due to licensing issues.