Not to many people know about this, and it is a parallel with the more well-known plan of extermination. If someone denies the latter, they will also have to find a way to rationalize away the evidence of the former.

"The plan was partially realized during the war, resulting directly and indirectly in the deaths of 9.4 to 11.4 million ethnic Slavs by starvation, disease, execution or extermination through labor, including 4.5 million Soviet citizens, 2.8 to 3.3 million Soviet POWs, 1.8 to 3 million Slavic Poles, 300 to 600 thousand Serbs and 20 to 25 thousand Slovenes. Its full implementation, however, was not considered practicable during the major military operations, and was prevented by Germany's defeat."

Hans Wilhelm Münch (14 May 1911 – c. 2001) [en.wikipedia.org]
"When someone says that Auschwitz is a lie, that it is a hoax, I feel hesitation to say much to him. I say, the facts are so firmly determined, that one cannot have any doubt at all, and I stop talking to that person because there is no use. One knows that anyone who clings to such things, which are published somewhere, is a malevolent person who has some personal interest to want to bury in silence things that cannot be buried in silence."
— Münch interviewed by the German filmmaker Bernhard Frankfurter

What is very troubling though is that the aforementioned genocide is much less known than what is known as the Holocaust, and, that those who decry the Holocaust never seem to mention it.

If you are against racism and genocide, you should not pick and choose, you oppose it all, not just the events that happened to target your ethnicity, and you should not be using it to support your own plans to isolate, destroy, and yes, exterminate, other races, and steal their land. That is not only the height of hypocrisy, it is also the epitome of extreme racism.