In the past on this subreddit, there have been really great answers discussing the impact that increased access to Soviet archives from Glasnot through the fall had on scholarship of World War II and the Eastern Front. Likewise there have also been some very interesting pieces written on how the Holocaust was understood within the Soviet Union, and especially how the aspect of Judeocide was in many ways downplayed in favor portraying it as more focused on the Soviet people generally.
So dwelling on both of these, I wonder what the impact was specifically on Holocaust historiography and understanding of the genocide in the East when Western scholars began to see the increased access that came about in the '90s, as well as the ability for both (former) Soviet and Western scholars to more closely collaborate, and the latter to write more openly and away from the party line.
To say "enormously" would be an understatement. In fact, this assessment isn't limited to the Holocaust but to every field of study touching on the former territory of the Soviet Union.
Regarding the Holocaust specifically, the effect was two-fold: with one aspect general and one specific. The general aspect had to do with the tendency among Soviet historians, under directives from above, to minimize the role of Jewish suffering during the war or the role of antisemitism as an ideology in motivating mass murder when committed by the Nazis as invaders of the USSR. Instead, there was a tendency to depict that Soviet people as a whole as having been the victims of "fascist invaders" or "Hitlerite Germans." On one hand, this depiction of Soviet suffering is wholly appropriate given the sheer level of suffering experience by Soviet people during the war; e.g., one in four villages in Belarus were destroyed by the Nazis, with a similar percentage of the general population similarly destroyed. On the other hand, there is little question that Jews as a population group within the Soviet Union were specifically targeted for murder -- first military aged men and boys and ultimately the whole population. The latter part was largely what the Soviet historiography sought to suppress. The Ukrainian poet Yevgeny Yevtushenko's famous poem "Babi Yar" (available here: https://remember.org/witness/babiyar) is famous in its condemnation of Soviet authorities for their willful forgetfulness.
In a more specific sense -- though closely related to the issue of Soviet de-Judaizing of wartime suffering -- the historiographic account of the Einsatzgruppen changed dramatically with the opening of Soviet archives. These special deployment groups, which followed the army into occupied territory and targeted pre-identified population groups on the basis of their real or perceived enemy status, we now know were responsible for at least one million and perhaps as many two million murders of Jews in incidents of mass shooting, which took place in two waves in 1941-1943. Before the Soviet archives were opened, it was thought that the death toll from these actions was perhaps half as small as it actually was. Moreover, the Jewish ancestry of the victims was again partially suppressed, although the task was more difficult in specific cases because the non-Jewish population had witnessed the murder of their Jewish neighbors and were there to tell anyone who would listen.
An interesting exercise is to look at how the war on the Eastern Front and its toll on the Jewish population changed over the course of time. Gerald Fleming's book Hitler and the Final Solution, although published before the fall of the USSR, relied fairly heavily on documents from the Soviet Union that had previously not been available; as a result, it offers a perspective on mass shootings in the Baltic States, e.g., that are both qualitatively and quantitatively different from what had come before. By the time the archives had been open for some time, the general picture of how the Nazis' policy regarding the Jewish population in eastern Europe came more to resemble what it is today.
The last point to make is that some of the important documents from the USSR had been available for longer than after 1991. Depending on the individual cases, when E. and W. Germany began to try war criminals in the 1960s, the USSR provided the prosecution with documents as needed. Perhaps the most famous case of this happening is the so-called Jäger Report, which details the murder of Lithuanian Jews in the fall of 1941 by Einsatzkommando 3. Although seized by the Soviets in 1945, the document was not known in the west until 1963, when it was introduced as evidence in the E. German case against Hans Globke, who bore some responsibility for contribution to the promulgation of the Nuremberg Laws.
For me at least, trying to comprehend its impacts feel almost immeasurable. I'm only 27, and have really only been comprehensively involved in researching the holocaust for about 4 years now - so my grasp the previous generations historiography is mostly limited to the classics. The best way to probably give a picture of its impacts would be to hyper focus on Auschwitz. As Arno J. Mayer put it in "Why Did the Heavens Not Darken?" - its also the origin for the the very term you used above actually ("Judeocide"): "Sources for the study of the gas chambers are at once rare and unreliable. . .The SS not only destroyed most camp records, which were in any case incomplete. . In the meantime, there is no denying the many contradictions, ambiguities, and errors in the existing sources." (Why did the heavens not darken? 1988)
It was only a year later that hobby historian Jean-Claude Pressac would write his magnum opus; "Auschwitz: Technique and Operation of the Gas Chambers". This book seriously improved our understanding of the final solution in Birkenau. It provided to the west much needed documentary materials from the construction archive, that were far from being readily available, that suggest previous understands of the crematoriums construction had been misguided. It used to be suggested crematorium 2 & 3 were constructed from the very beginning for the extermination of prisoners. (Propagated by historians like George Wellers.) Which we now know to be inaccurate. (http://holocaustcontroversies.blogspot.com/2018/12/a-new-document-mentioning-special.html)
While also providing multiple "criminal traces" (https://phdn.org/archives/holocaust-history.org/auschwitz/pressac/technique-and-operation/pressac0429.shtml) that corroborated the testimony of multiple perpetrators and sonderkommando. Now this isn't to suggest that the soviets weren't slowly releasing materials to the west. Two such examples are Jäger Report's report (in 1963) and John Demjanjuk's identity card. But it was far from being satisfactory.
Perhaps the most important area the Soviet Union's fall has opened up is archaeology. It would honestly be impossible to summarize the work of Andrzej Kola, Caroline Sturdy Colls, or Michael Tregenza. Their research has helped western historiography reconstruct the perimeters of Belzec, Sobibor, and Treblinka as well as the camps mass graves - less so at Treblinka though. Or see Patrick Desbois's research into the "Holocaust of Bullets" which killed nearly 1.15 million Jews.
It's also greatly increased our knowledge into collaboration in the Soviet Territories as well. Such as OUN-UNP, LAF militias, and even Poles in locations like Jedwabne. And these collaborators home countries are now trying to downplay their crimes while simultaneously make them national heroes. (http://defendinghistory.com/)
Basically it's increased our collective knowledge in all fields.