If the definition of this Holocaust is the deaths of 6 million Jews…then doesn't the amending of deaths at Auschwitz from 4 million to 1.5 million make it all open to debate?

All of a sudden instead of 6 million we have 3.5 million. Is it still technically "the" Holocaust?

You are mixing up two different things. Holocaust historiography found itself tangentially involved with the Cold War, so Western and Soviet historians didn't exactly see eye to eye for quite some time. The estimate of 6 million Jews was reached by Western historians, while the estimate of 4 millions in Auschwitz was done by Soviet historians, and neither side took the other side's number into account. Turns out that Soviet historians inflated their numbers in part because of the Cold War: since the US had rearmed West Germany it was convenient for the USSR to remind everyone of just how bad an idea that was.

Once the Cold War ended and historians stopped being divided by the Iron Curtain, exchange of ideas and historical records began anew. It turned out that the "6 million Jews" number was more or less correct, while the "4 million Jews at Auschwitz" wasn't. But since the latter wasn't taken into account to calculate the former, only the latter had to be emended.

In what other instance has the discussion or debate of an alleged historical event been made a crime?

The Holocaust is open to honest debate by actual Historians. When you look closely at deniers and "revisionists" it's almost certain that they have about as much qualifications to be historians as Kanye West. And when you look at them even more closely, they almost always turn out to be either Neo-Nazis trying to whitewash their ideology or Antisemites trying to further defame the victims. Since "revisionist" (i.e. not done by actual historians) Holocaust "debate" ends up being an attempt to deny established facts as a way to propagate hateful and anti-democratic ideologies, it stands to reason that democracies will try to keep close tabs on it to protect democratic values.