It seems to me that, in popular consciousness, when people think of Nazi war crimes or genocide what's thought of is the Holocaust and the Holocaust alone. Why is it that Generalplan Ost is less recognized, despite being its own genocide resulting in the deaths of millions? Is it because of general lack of understanding of the Eastern Front? Does this lack of understanding help contribute to "clean Wehrmacht" myths?
This might be a question better answered through historiography, but I would venture that it has more to do with the actual execution of Generalplan Ost and the Final Solution. The Einsatzgruppen -- or mobile death squads -- targeted both Jews as well as ethnic Slavs. While the planned to eliminate the Jews through slave labor and mass murder, the official stance on the Slavs is less definite.
Chaim Rumkowski, the leader of the Nazi-appointed Judenrat in the Łódź ghetto is perhaps one of the most famous Jewish collaborators.
His "Give me your children" speech is perhaps one of the most chilling tracts to come from any ghetto:
They are asking us to give up the best we possess—the children and the elderly. I never imagined I would be forced to deliver this sacrifice to the altar with my own hands. In my old age, I must stretch out my hands and beg. Brothers and sisters: Hand them over to me! Fathers and mothers: Give me your children!
(https://encyclopedia.ushmm.org/content/en/article/give-me-your-children-voices-from-the-lodz-ghetto)
Rumkowski also relied on his own police force and requested the Nazis to violently suppress demonstrations against his administration in the ghetto. Despite his collaboration, Rumowski died at Auschwitz following the final deportations from Łódź. The General Government in Poland, however, looked to reestablish a hegemony like that exerted by Prussia following the partitions, albeit far more brutal one. In other words, Poles, Ukrainians, and Russians had some opportunity for survival, but even the most conciliatory Jewish collaborators were slated for the death camps.
It's worth noting that the SS tested Zyklon B on Russian soldiers before implementing it in the gas chambers. This anecdote comes from The Destruction of the European Jews by Raul Hilberg, which is pretty much the gold standard for any monograph in US Holocaust Studies. The genocidal policies aimed at the Jews and Slavs are distinct but linked.
To return to your initial question, I think there's a case to be made that the Nazis conducted a genocide against the Slavs. (I think the "clean Wehrmacht" myth you mentioned also plays a minor role in a popular conception of the Holocaust.) The OED defines a genocide as "the deliberate and systematic extermination of an ethnic or national group." Nazi policy against the Jews fits squarely under this definition. There's really no room for debate. The targeted murders carried out by Einsatzgruppen in Poland and the Soviet Union certainly fit under the qualifiers "deliberate" and "systematic," but whether these would have culminated in outright extermination is less clear. The speculative answers range between maybe and probably, but the ultimate goals for Slavs under Nazi racial policy were more flagrant than they were meticulous.
A very informative answer to a similar question was written by u/commiespaceinvader here.
A helpful takeaway from that answer:
"... the Generalplan Ost had in the entirety of the plans it laid out, genocidal consequences, calling for the removal and the killing of all in all 30 million people from all over Europe to make space for 12 million German settlers. But while the GPO showcases the genocidal nature of the Nazi regime in yet another one of its facets, unlike the Holocaust, meaning the orgnaized, state-driven and sponsored program to murder Europe's Jews and so-called Gypsies, it was never implemented in this genocidal entirety.
What was implemented was very piecemeal and compared to the grand vision of the plans, comparatively small scale. Several of the connected actions, especially the Operation Tanneberg, Intelligenzaktion and the A-B Aktion were genocidal in nature as they were directed against who the Germans perceived as the "carriers of the Polish nation" and implementations like the Aktion Zamosc are war crimes and crimes against humanity to boot.
The reason, they are not commonly included in the term Holocaust however, is the practical and structural difference between the systematic, bureaucratic, state-sponsored persecution and murder of six million Jews and up to half a million Roma, Sinti, and other groups persecuted as "gypsies" by the Nazi regime and its collaborators and these actions."
Though the answers that have already have been posted have been great, I would like to point OP and anyone who is interested to the work of Isabel Heinemann, whose work (though most of its in German) can give you more detail on SS ethnographers who did put precise numbers on the proposed extermination of "sub-humans" as part of the larger population redistribution which was part of General Plan Ost. It should be noted that these redistribution plans as mentioned by others were specifically pointed towards different nationalities, rather than a larger "slavic" race. One specific though brief resource in English is her chapter in Bartov's Holocaust: Origins, Implementation, Aftermath, 2nd Edition (See ciation below for more information on that book chapter)
Heinemann, I. (2015). Defining “Unwanted Population Addition.” In O. Bartov (Ed.), The Holocaust: Origins, Implementation, Aftermath (2nd ed., pp. 42–56). New York, NY: Routledge.