Allies were getting reports of what was going on but chose not to publish anything. However they knew what was happening. For the Soviets, it was obvious what was going on when they recaptured territory but of course seeing as they truly lose the war in Fatherland their opinion at least in the book was less important. At one point Charlie states that the Rump USSR has all the evidence it needs, it's trying to convince the Americans that is the problem.
OTL western powers
suspected what was happening.
Evidence of a crime being committed in enemy territory is difficult to have, and could be easily discarded as war time propaganda by the public reading newspapers.
Even the government and its intelligence service has to be very careful in screening information coming from rebels in enemy countries and/or spies, since often it is grossly inaccurate, both for technical reasons (rebels not often have an intelligence training, enemy counter-intelligence is at work, some people is just incompetent), and for willing misleading (an agent telling something big to gain prestige in the service, a cooperator doing the same to have money, rebels do the same to incite the foreign government to take active military measures, etc.).
Regarding TTL rump soviet union, if I remember the book, it was spurned by the whole war as a mass-murder society (red terror, katyn, de-kulakization, collectivization famine, Moscow trials), and held pretty much the same position a Nazi germany would held in OTL.
IIRC, there was a line about an entire branch of history being about the crimes of stalinism.
In a few words, not someone you would trust as a witness of someone other's crime