The name has probably been there since the war or since the stadium was built, if that was later. A lot of the Partisans were Serbs. The Red Army drove the Germans out of Belgrade in October 1944, 70 years ago, and Tito & company ran things from then on. Only people 80+ would have opinions based on what they knew or experienced in the war, while the rest were fed pro-Tito propaganda growing up. I doubt many people in Belgrade have any problem with the name of the stadium.