Ah, shit. The matter of whether the Holodomor should or should not bear the label of 'genocide' remains an unresolved question in academic circles, so...ergh. As this has come up before, here's some previous threads on the matter:
As always, if anyone would like to add their own input, please feel free to do so!
With one caveat, as I must now speak with the green Hat of Moderation on: Given the entities at play and current events, all are reminded that we have a 20-Year Rule. Further, while there is debate around how intentional it was, it definitely did happen - and genocide denialism is a violation of our civility rule and an autoban. Mind your manners, anyone wanting to respond.
The mod comment hit it on the head. It is an ongoing question in academic circles, but I want to address another part of your question.
I am in the camp with Anne Applebaum in stating that the famines were largely a result of a combination of extending traditionally Russian collective farming institutions where they did not already exist, and violent efforts to destroy "Kulaks" by framing them as parasites and capitalist oppressors while leaving the actual definition of who was and was not a Kulak entirely up to local authorities. There was also a failure of the Communist Party structure that resulted in higher crop yield being reported than were actually harvested, leading the Soviet government to essentially believe their own propaganda. On top of all this, there were restrictions on the use of the Ukranian language. There is a conflict at every level of Ukranian ethnic identity here (to say nothing of the Soviet treatment of Tatar people in the region. I do consider that genocide regardless of intent.
On one hand, it wasn't genocide in the sense that the famines resulted from truly incompetent management and over zealous centralization. In effect, it was an unintentional accident. On the other hand it was genocide because the Holodomor resulted from a sort of colonialism of economic institutions and an attempt to russify Ukraine, and was thus structural in nature and does not require intent. Regardless of my opinion, I can certainly understand the debate.
However, the point that I actually want to address is why r/ worldnews is banning genocide denial. I will talk a bit about current events here, but it is necessary to talk about the broader discourse on genocide denialism.
The current Russian war effort in Ukraine is deeply nationalist and rooted in irredentist claims on Ukranian land and revanchist sentiments towards NATO. That irredentism is justified through a flat out denial of Ukranian national and ethnic identity. Vladimir Putin himself has referred to the Ukranians as no more than little Russians, which is a flat out denial of Ukranian national identity. The denial of an ethnic or national identity is itself an excuse to marginalize or suppress that ethnic expression. Putin gave a speech to this effect during the start of the current invasion.
Denying the Holodomor is crucial to justifying this attitude and the current war effort.