Jan Karski went to UK and US in 1943 where he met with top politicians including Churchill, Roosevelt and Hull, also with tons of officials, military brass, celebrities and other social parasites, showing them proof of holocaust happening. And was almost completely ignored. He did not had the full info though, and it was before the holocaust went in the full extent, but still the systematic extermination of Jews and other undesirables was evident.

You’re right though that only the Soviet offensive allowed for full extent of holocaust to be revealed, and nazis were often making effort to hide their crimes before they retreated, so a lot of proof was probably destroyed, even though the remaining was still enough to make holocaust the best documented genocide in history.

Of course the full scale was unknown, but even according to documents and accounts, it was sometimes an open secret.