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In Return of the Jedi Mon Mothma says in her briefing of the rebel troops
Many Bothans died to bring us this information.
But the team of people who went the the information base were almost entirely human, and I didn’t see any bothans among their ranks. Now I know the Bothans are stealthy, but in order to sneak past the audience there has to be a real reason they weren’t shown.
"Many Bothans died" line was not from Episode IV ("Star Wars: A New Hope") which Rogue One is a prequel of, referring to Death Star 1.
The only thing we know about how Rebels got the plans from ANH is in the opening crawl:
Rebel spaceships, striking from a hidden base, have won their first victory against the evil Galactic Empire. During the battle, Rebel spies managed to steal secret plans to the Empire’s ultimate weapon, the DEATH STAR, an armored space station with enough power to destroy an entire planet. Pursued by the Empire’s sinister agents, Princess Leia races home aboard her starship, custodian of the stolen plans that can save her people and restore freedom to the galaxy….
as well as Vader's assertion that Tantive IV intercepted "a transmission".
Instead, it was in Episode VI ("Return of the Jedi") - note Lando standing next to her, in case you need proof; referring to information about "unfinished" Death Star 2 and Emperor's plans to visit there, ~4 years past the events of A New Hope and Rogue One.
Mon Mothma is referring to the second Death Star. Neither she nor Akbar make mention of new blueprints although we can safely assume they have them.
Specifically regarding the information the Rebels got from the Bothans, Mon Mothma says 3 things.
She finishes by saying that "Many Bothans died to bring us this information"
Akbar then takes over the briefing to talk about the defense shield on the Forest Moon of Endor, says that the Rebel cruisers will create a perimeter and the fighters will enter the Death Star to go after the reactor.
They seem to be employing the same strategy (more or less) as the did in Episode IV (blow the reactor) they just have a clearer shot at it since the Death Star II isn't complete and there are passages large enough to get a starship through.
That said, to answer the question that was asked, the Bothans seemed to come up between Episode v and Episode VI specifically to find out where the Death Star II was being constructed and its current state of completion. That the emperor was on board was an added bonus.
Edit: Here's part of the scene. Apologies for video quality
