Bethesda did not remember to remove the Denuvo-less .EXE from the game's package.
As players were anxiously waiting for Doom Eternal to unlock, a few noticed something particularly strange in the game's files. The install folder had the usual, main .EXE file, but also another one untouched by the Denuvo DRM.
The unmodified file, for some reason, was left in the downloaded package, under a folder called "original" no less. As I am sure you know by now, the whole point of Denuvo DRM is to thwart piracy at launch, but that doesn't work if anyone could freely access the game without said DRM.
Resetera forum members discovered the oopsie in the Bethesda Launcher version of the game, with the Steam version seemingly unaffected. Shortly after Doom Eternal went live, a patch was released to remove the unmodified .EXE file, but not before pirates took notice.
This also appears to be something Bethesda has done before with Rage 2, where it forgot to ship the DRM-enabled .EXE in the Bethesda Launcher version, once again making the job of pirates easier.
Doom Eternal is out today on PC, PS4, Xbox One and Stadia. A Switch version is coming in 2020.