It starts well before Soviet times: Wikipedia says the Ukrainian language was banned in the Russian Empire starting in 1804 when what is now Eastern, Central and Southern Ukraine were part of the Russian Empire.
That is true, and while these bans had a huge historical impact, they didn't change the language that was used in villages and farms.
Until the 1920s Ukraine was a mostly rural country and in the countryside Ukrainian was alive and well. After the wars and insurgency the Soviet government felt compelled to create a Ukrainian SSR and allow widespread use of Ukrainian. Most towns and cities spoke Russian (and also German, Yiddish and Polish), but then as industrialisation forced many peasants into urban areas, they quickly became Ukrainian-speaking. In this context a new wave of language repression began, which became really intense only in the 1960s and 1970s. Of course there other forms of repression besides language
Until the 1920s Ukraine was a mostly rural country and in the countryside Ukrainian was alive and well. After the wars and insurgency the Soviet government felt compelled to create a Ukrainian SSR and allow widespread use of Ukrainian. Most towns and cities spoke Russian (and also German, Yiddish and Polish), but then as industrialisation forced many peasants into urban areas, they quickly became Ukrainian-speaking. In this context a new wave of language repression began, which became really intense only in the 1960s and 1970s. Of course there other forms of repression besides language