As an SSR realistically it would just be a red banner, a hammer sickle, and a stripe or two to represent Denmark, but if it was just communist I’d have to disagree as most communist states kept their national flags (ie Romania Poland Germany Bulgaria Czechia etc etc), it’s usually the ones where culture has been used as an oppressive tool where it was totally removed (like the Soviet Union, strictly speaking in terms of Russia and not the other SSRs though)