In order to understand this shift, we first need to understand why anti-slavism came into the National-Socialist German Weltanschauung. There are two main reasons for this: Territorial and Racial.
On the territorial part, it's quite simple and straightforward: Since it's inception National-Socialist ideology considered the German people Volk ohne Raum, people without space - This was also part of 25 points National Socialist Program adopted by Hitler in 1920: "We demand land and soil (colonies) for the feeding of our people and the settlement of our population surplus.". They viewed the reduction of German territory after WW1 in favor of Slavic territories (namely Poland) as an affront, as Germany was the most populous west European country. They viewed the sparsely populated Eastern territories of the Soviet Union as desirable for conquest and colonization by the space deprived German people - crystallized in the 19th century concept of Drang nach Osten, the push to the east, which predates even the formation of Germany, and draws its roots on the Teutonic Crusades.
Since the end of WW2 however, the dependence on Agricultural and Industrial production has faded, leading to the abandonment of the eastwards expansion of Germany in the German National-Socialist ideology. As for the non-german National-Socialist ideologies (which expanded after WW2), this concept was never prioritary, as it is related to German geographical positioning. So it makes no sense for a British National-Socialist, for example, to defend an expansion eastward.
On the racial part, we need to analyse the conditions that lead to Slavs being considered Untermenschen, subhumans. Contrary to popular belief, the concept of Untermensch is not related only to genetic or racial characteristics, but also to the behavior of an individual, meaning an Aryan person could be considered inferior if he demonstrated behavioral tendencies associated with Untermensch, like support of Communism. As Streicher wrote in 1938, in relation to the Bavarian Soviet Republic: "It was the time of the [Bavarian] Soviet rule. As the liberated Untermenschen [the communists] marched through the streets, the deputies hid behind a fireplace in the Bavarian parliament."
Goebbels stresses that the concept of subhumanism is not restricted to race itself, but also prevalent on every race, as a catalyst for the expansion of anti-Nationalist policies: "(...) the Untermenschentum**,** that exists in every people, acts as yeast (...)" As such, the Slavic populations of the Soviet Union where considered Untermenschen not due to their racial characteristics (as they were considered Arier or Nordmongolien (like the finnish) according to the Meyers Konversationslexikon of 1890), but due to their support of Communism and willingness to follow the Jewish Bolshevik elite who ruled the Soviet Union at the time. Slavic children under 10 were considered non-indoctrinated and fit for germanization, and were to be sent to german families. Similarly, slavic supporters of National Socialism (or even just opponents of Communism) were accepted in the German society and even fought in the Waffen-SS and Wehrmacht in large numbers.
Since the fall of the Soviet Union and other Communist eastern european countries however, this racial reason has became outdated - there is no reason to consider the Slavic people as having a submissible response to Communism anymore, thereby removing them from the behavioral Untermenschen category.
Resuming, anti-slavism is absent from modern National-Socialist ideology due to the removal of the reasons that lead to it.
Sources:
Essays in German history, Francis Ludwig Carsten (Book)
25-Punkte-Programm der NSDAP (in German) (https://web.archive.org/web/20140719024113/http://www.dhm.de/lemo/html/dokumente/nsdap25/)
Deutsche und Polen: Geschichte, Kultur, Politik, Andreas Lawaty & Hubert Orlowski (Book)
Der Stürmer article Kampf dem Weltfeind, Julius Streicher (Newspaper)
Cruel World: The Children of Europe in the Nazi Web, Lynn Nicholas (Book)