Probably in lots of little pieces
Tarkin suggested that the loss of the plans would not be a great setback:
Tarkin spoke with detached consideration, as much to himself as Romodi. “The original plans for this station are kept at the Citadel, are they not?”
“They are.”
Along with other technical schematics for projects covered by the Tarkin Initiative. It would be a special pity, Tarkin thought, to see War-Mantle and Stellarsphere set back. But hardly a major blow to the galactic timetable, particularly with the Death Star finally online.
Rogue One: A Star Wars Story
He also refers to the “original plans” being kept on Scarif, suggesting that there are modified or partial versions elsewhere. This only makes sense, of course: some portion of the plans would be necessary for the actual construction of the Death Star, which wasn’t happening on Scarif. Indeed, Galen Erso noted that the engineering archive on Scarif was complete:
“I know there’s at least one complete engineering archive in the data vault at the Citadel Tower on Scarif. Use what I’ve told you, run the analysis, and you’ll be able to plan your attack. Any pressurized explosion to the reactor module will set off a chain reaction that will—”
Rogue One: A Star Wars Story
It’s possible that there was another complete archive that Galen did not know about, but this seems improbable. That the full plans were not available to most people seems likely: the Emperor purposely kept construction of his superweapon fragmented.
All the same, he had no compelling reason to solve the enigma of the battle station’s beginnings. What bothered him was that, compliant with a strategy that no base commander—Moff, admiral, or general—should have unrestricted access to information regarding shipments, scheduling, or construction progress, no single person was in charge of the project, unless of course the Emperor was considered to be that person.
Tarkin
But there would have been numerous partial plans at the various Imperial research facilities (such as the one Galen worked at). In time, the project could be reconstructed from those, and from the expertise of the people who had worked on it (as could, apparently, War-Mantle and Stellarsphere).1
1: That these projects would have been set back suggests that, for them at least, there were not other complete schematics readily lying around.