There was an ebb and flow to it, like everything. In the lead up to WWII as fascism was a growing existential threat to the Soviet Union, speech become constricted in an effort to prevent subversion. (Also, there was an ongoing cultural revolution against bourgeois influence) Even the USA enacted similar restrictions once they finally chose a side and joined the war, because in a time of existential total war governments typically suspend some speech rights for obvious reasons. But from the 50s onward, there were much fewer restrictions. George Lucas is even on record praising the degree of creative freedom Soviet directors had over their approved projects in the 70s.

If Blackshirts and Reds was anything to go by, they didn't censor enough.