Following the release of Doom, Doom 2 and Doom 3 on Xbox One after Quakecon this weekend, the old Xbox 360 games were not only delisted from the marketplace, but made inaccessible to download by those who already owned them.
Bethesda has now fixed the latter problem, however, and confirmed in a tweet that if you own the Xbox 360 releases you can redownload them on Xbox One.
The games are still not purchasable if you did not own them already.
Source: Eurogamer Bethesda fixes issue where Doom Xbox 360 owners couldn't redownload game