For whatever reason you want it to, WWII ends in a Fatherland-esque scenario. How easy is it for the Nazis to keep plausible deniability surrounding the Holocaust?
They wouldn't keep it a secret. They'd be hailing it as a great achievement for humanity, like curing cholera and rabies.
IOTL anything connected to Bletchley Park was kept secret until the 1970s, thirty years after the regime they’d been listening to had ceased to exist; if the Nazi regime survived and dominated continental Europe, Bletchley Park would remain the best kept secret of the war for the duration of any subsequent anti-Nazi Cold War. Since nothing as to a source for reports of the Holocaust could be revealed without revealing Bletchley, it would either never be made public or it would be discounted by the public as baseless war propaganda. Prior to the film footage from Belsen been shown in cinemas at the end of the war, many people had thought the stories where merely propaganda.As to when it was know, we were sure no later than 1943 when we intercepted a status report with the number killed, seem like over 3 million in the message. So it would have always been known.
They wouldn't keep it a secret. They'd be hailing it as a great achievement for humanity, like curing cholera and rabies.
Having said that, because it would not come out in a rush, no Nuremburg trials, etc, the impact of this would be much smaller.
It would depend on the scale of what happened and when. The fact of the holocaust was reported in Western media in 1942, but of course with out the visuals we saw in otl