Up until the cold war era, it was accepted that it was the Nazis who did it. You can even find old articles online from the NYT of all places claiming that the whole thing was a lie perpetrated by Goebbels:
During the cold war, they did a 180° and the Nazi narrative was then accepted without question. In the 90s, Gorbachev "admitted" to it, but couldnt produce any proof or point to any perpetrators, plus he's Gorbachev. The people repeating this version of the story now are all the usual suspects: NATO, bourgeois journalists and Polish nationalists. It's definitely part of anti-communist and Polish nationalist mythology now.
Goebbels himself didnt actually know who did it when the mass graves were found in 1943, but a supposed entry on his diary reveals that he was worried about the fact that the ammo casings found on the location were all German-made. Nevertheless, he blamed the soviets for propaganda purposes.
Were the soviets using german ammunition, for whatever reason? Is the diary a forgery? The liberals will usually claim one of those options. The 2010s investigations done by russia were not well received, and they were also inconclusive.
I do think a lot of people here are too quick to scream "they were nazi collaborators!" when the soviets themselves claimed they didnt do it, and when the evidence is sparse and almost always politically charged. I also think this is caused by counterjerking too hard without looking at the facts.