Hi, this is wrong. Whoever read anything about the Bosnian war knows this is false. I will try to give you a short recap of this situation:

Bosnia was a country made from Serbs, Croats and Muslims (later renamed Bosniaks), and it was an inner civil war there after the breakup of Yugoslavia, which was supported indirectly by Serbia and Croatia, but without direct involvement of these countries' armies. The reason for the war is that Muslims wanted to separate Bosnia from Yugoslavia, Croats to separate part of Bosnia and unite with Croatia (Herzeg-Bosnia), while Serbs wanted to stay with Serbia in the same country. The ethnicities were mostly mixed throughout the whole territory, which meant it wasn't possible to make any clear cuts between them, but war was led in order to capture as much as territory as possible and remove other etnicities from it. In the end the international community intervened through NATO and later diplomacy and made a compromise solution between the 3 ethnicities by keeping the country as whole, but creating 3 different administrative entities (FBIH, RS, and Brčko District), and making the country independent. This is the state of the country today. Although there is peace for over 20 years now, Muslims and Serbs are still in very bad relations, as Serbs feel that Muslims prevented them to unite with their people, while Muslims feel that Serbs and Croats prevented them to take RS territory as their own and create a unitary Bosnia.

This doesen't mean Nurkić doesn't have the right to wear the shirt. He has every right to express his political opinion about the Bosnian war if NBA rules allow this. I personally find it distasteful as the only Serb on the court was Jokić and it can be interpreted as some kind of provocation, and also Serbs cheering for Portland won't find this very nice. Other than that, I see no issue.